Linaro-mm-sig October 2025

linaro-mm-sig@lists.linaro.org

27 participants
60 discussions

Re: [RFC v2 0/8] dma-buf: Add support for mapping dmabufs via interconnects

by Jason Gunthorpe

On Thu, Oct 30, 2025 at 06:17:11AM +0000, Kasireddy, Vivek wrote: > It mostly looks OK to me but there are a few things that I want to discuss, > after briefly looking at the patches in your branch: > - I am wondering what is the benefit of the SGT compatibility stuff especially > when Christian suggested that he'd like to see SGT usage gone from > dma-buf I think to get rid of SGT we do need to put it in a little well defined box and then create alternatives and remove things using SGT. This is a long journey, and I think this is the first step. If SGT is some special case it will be harder to excise. So the next steps would be to make all the exporters directly declare a SGT and then remove the SGT related ops from dma_ops itself and remove the compat sgt in the attach logic. This is not hard, it is all simple mechanical work. This way the only compat requirement is to automatically give an import match list for a SGT only importer which is very little code in the core. The point is we make the SGT stuff nonspecial and fully aligned with the mapping type in small steps. This way neither importer nor exporter should have any special code to deal with interworking. To remove SGT we'd want to teach the core code how to create some kind of conversion mapping type, eg exporter uses SGT importer uses NEW so the magic conversion mapping type does the adapatation. In this way we can convert importers and exporters to use NEW in any order and they still interwork with each other. > eventually. Also, if matching fails, IMO, indicating that to the > importer (allow_ic) and having both exporter/importer fallback to > the current legacy mechanism would be simpler than the SGT > compatibility stuff. I don't want to have three paths in importers. If the importer supports SGT it should declare it in a match and the core code should always return a SGT match for the importer to use The importer should not have to code 'oh it is sgt but it somehow a little different' via an allow_ic type idea. > - Also, I thought PCIe P2P (along with SGT) use-cases are already well handled > by the existing map_dma_buf() and other interfaces. So, it might be confusing > if the newer interfaces also provide a mechanism to handle P2P although a > bit differently. I might be missing something here but shouldn't the existing > allow_peer2peer and other related stuff be left alone? P2P is part of SGT, it gets pulled into the SGT stuff as steps toward isolating SGT properly. Again as we move things to use native SGT exporters we would remove the exporter related allow_peer2peer items when they become unused. > - You are also adding custom attach/detach ops for each mapping_type. I think > it makes sense to reuse existing attach/detach ops if possible and initiate the > matching process from there, at-least initially. I started there, but as soon as I went to adding PAL I realized the attach/detach logic was completely different for each of the mapping types. So this is looking alot simpler. If the driver wants to share the same attach/detach ops for some of its mapping types then it can just set the same function pointer to all of them and pick up the mapping type from the attach->map_type. > - Looks like your design doesn't call for a dma_buf_map_interconnect() or other > similar helpers provided by dma-buf core that the importers can use. Is that > because the return type would not be known to the core? I don't want to have a single shared 'map' operation, that is the whole point of this design. Each mapping type has its own ops, own types, own function signatures that the client calls directly. No more type confusion or trying to abuse phys_addr_t, dma_addr_t, or scatterlist for in appropriate things. If your driver wants something special, like IOV, then give it proper clear types so it is understandable. > - And, just to confirm, with your design if I want to add a new interconnect/ > mapping_type (not just IOV but in general), all that is needed is to provide custom > attach/detach, match ops and one or more ops to map/unmap the address list > right? Does this mean that the role of dma-buf core would be limited to just > match and the exporters are expected to do most of the heavy lifting and > checking for stuff like dynamic importers, resv lock held, etc? I expect the core code would continue to provide wrappers and helpers to call the ops that can do any required common stuff. However, keep in mind, when the importer moves to use mapping type it also must be upgraded to use the dynamic importer flow as this API doesn't support non-dynamic importers using mapping type. I will add some of these remarks to the commit messages.. Thanks! Jason

11 hours, 50 minutes

Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO regions

by Leon Romanovsky

On Wed, Oct 29, 2025 at 05:25:03PM -0700, Samiullah Khawaja wrote: > On Mon, Oct 13, 2025 at 8:27 AM Leon Romanovsky <leon(a)kernel.org> wrote: > > > > From: Leon Romanovsky <leonro(a)nvidia.com> > > > > Add support for exporting PCI device MMIO regions through dma-buf, > > enabling safe sharing of non-struct page memory with controlled > > lifetime management. This allows RDMA and other subsystems to import > > dma-buf FDs and build them into memory regions for PCI P2P operations. > > > > The implementation provides a revocable attachment mechanism using > > dma-buf move operations. MMIO regions are normally pinned as BARs > > don't change physical addresses, but access is revoked when the VFIO > > device is closed or a PCI reset is issued. This ensures kernel > > self-defense against potentially hostile userspace. > > > > Signed-off-by: Jason Gunthorpe <jgg(a)nvidia.com> > > Signed-off-by: Vivek Kasireddy <vivek.kasireddy(a)intel.com> > > Signed-off-by: Leon Romanovsky <leonro(a)nvidia.com> > > --- > > drivers/vfio/pci/Kconfig | 3 + > > drivers/vfio/pci/Makefile | 2 + > > drivers/vfio/pci/vfio_pci_config.c | 22 +- > > drivers/vfio/pci/vfio_pci_core.c | 28 ++ > > drivers/vfio/pci/vfio_pci_dmabuf.c | 446 +++++++++++++++++++++++++++++ > > drivers/vfio/pci/vfio_pci_priv.h | 23 ++ > > include/linux/vfio_pci_core.h | 1 + > > include/uapi/linux/vfio.h | 25 ++ > > 8 files changed, 546 insertions(+), 4 deletions(-) > > create mode 100644 drivers/vfio/pci/vfio_pci_dmabuf.c <...> > > +void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked) > > +{ > > + struct vfio_pci_dma_buf *priv; > > + struct vfio_pci_dma_buf *tmp; > > + > > + lockdep_assert_held_write(&vdev->memory_lock); > > + > > + list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) { > > + if (!get_file_active(&priv->dmabuf->file)) > > + continue; > > + > > + if (priv->revoked != revoked) { > > + dma_resv_lock(priv->dmabuf->resv, NULL); > > + priv->revoked = revoked; > > + dma_buf_move_notify(priv->dmabuf); > > I think this should only be called when revoked is true, otherwise > this will be calling move_notify on the already revoked dmabuf > attachments. This case is protected by "if (priv->revoked)" check both in vfio_pci_dma_buf_map and vfio_pci_dma_buf_attach. They will prevent DMABUF recreation if revoked is false. VTW, please trim your replies, it is time consuming to find your reply among 600 lines of unrelated text. Thanks > > + dma_resv_unlock(priv->dmabuf->resv); > > + } > > + dma_buf_put(priv->dmabuf); > > + } > > +}

12 hours, 36 minutes

Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO regions

by Leon Romanovsky

On Wed, Oct 29, 2025, at 18:50, Alex Mastro wrote: > On Mon, Oct 13, 2025 at 06:26:11PM +0300, Leon Romanovsky wrote: >> + /* >> + * dma_buf_fd() consumes the reference, when the file closes the dmabuf >> + * will be released. >> + */ >> + return dma_buf_fd(priv->dmabuf, get_dma_buf.open_flags); > > I think this still needs to unwind state on fd allocation error. Reference > ownership is only transferred on success. Yes, you are correct, i need to call to dma_buf_put() in case of error. I will fix. Thanks > >> + >> +err_dev_put: >> + vfio_device_put_registration(&vdev->vdev); >> +err_free_phys: >> + kfree(priv->phys_vec); >> +err_free_priv: >> + kfree(priv); >> +err_free_ranges: >> + kfree(dma_ranges); >> + return ret; >> +}

1 day, 7 hours

Re: [PATCH v1] drm/sched: fix deadlock in drm_sched_entity_kill_jobs_cb

by Christian König

On 10/29/25 10:11, Pierre-Eric Pelloux-Prayer wrote: > https://gitlab.freedesktop.org/mesa/mesa/-/issues/13908 pointed out > a possible deadlock: > > [ 1231.611031] Possible interrupt unsafe locking scenario: > > [ 1231.611033] CPU0 CPU1 > [ 1231.611034] ---- ---- > [ 1231.611035] lock(&xa->xa_lock#17); > [ 1231.611038] local_irq_disable(); > [ 1231.611039] lock(&fence->lock); > [ 1231.611041] lock(&xa->xa_lock#17); > [ 1231.611044] <Interrupt> > [ 1231.611045] lock(&fence->lock); > [ 1231.611047] > *** DEADLOCK *** > > My initial fix was to replace xa_erase by xa_erase_irq, but Christian > pointed out that calling dma_fence_add_callback from a callback can > also deadlock if the signalling fence and the one passed to > dma_fence_add_callback share the same lock. > > To fix both issues, the code iterating on dependencies and re-arming them > is moved out to drm_sched_entity_kill_jobs_work. > > Suggested-by: Christian König <christian.koenig(a)amd.com> > Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer(a)amd.com> Reviewed-by: Christian König <christian.koenig(a)amd.com> > --- > drivers/gpu/drm/scheduler/sched_entity.c | 34 +++++++++++++----------- > 1 file changed, 19 insertions(+), 15 deletions(-) > > diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c > index c8e949f4a568..fe174a4857be 100644 > --- a/drivers/gpu/drm/scheduler/sched_entity.c > +++ b/drivers/gpu/drm/scheduler/sched_entity.c > @@ -173,26 +173,15 @@ int drm_sched_entity_error(struct drm_sched_entity *entity) > } > EXPORT_SYMBOL(drm_sched_entity_error); > > +static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f, > + struct dma_fence_cb *cb); > + > static void drm_sched_entity_kill_jobs_work(struct work_struct *wrk) > { > struct drm_sched_job *job = container_of(wrk, typeof(*job), work); > - > - drm_sched_fence_scheduled(job->s_fence, NULL); > - drm_sched_fence_finished(job->s_fence, -ESRCH); > - WARN_ON(job->s_fence->parent); > - job->sched->ops->free_job(job); > -} > - > -/* Signal the scheduler finished fence when the entity in question is killed. */ > -static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f, > - struct dma_fence_cb *cb) > -{ > - struct drm_sched_job *job = container_of(cb, struct drm_sched_job, > - finish_cb); > + struct dma_fence *f; > unsigned long index; > > - dma_fence_put(f); > - > /* Wait for all dependencies to avoid data corruptions */ > xa_for_each(&job->dependencies, index, f) { > struct drm_sched_fence *s_fence = to_drm_sched_fence(f); > @@ -220,6 +209,21 @@ static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f, > dma_fence_put(f); > } > > + drm_sched_fence_scheduled(job->s_fence, NULL); > + drm_sched_fence_finished(job->s_fence, -ESRCH); > + WARN_ON(job->s_fence->parent); > + job->sched->ops->free_job(job); > +} > + > +/* Signal the scheduler finished fence when the entity in question is killed. */ > +static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f, > + struct dma_fence_cb *cb) > +{ > + struct drm_sched_job *job = container_of(cb, struct drm_sched_job, > + finish_cb); > + > + dma_fence_put(f); > + > INIT_WORK(&job->work, drm_sched_entity_kill_jobs_work); > schedule_work(&job->work); > }

1 day, 12 hours

Re: [RFC v2 0/8] dma-buf: Add support for mapping dmabufs via interconnects

by Jason Gunthorpe

On Wed, Oct 29, 2025 at 11:25:34AM +0200, Leon Romanovsky wrote: > On Tue, Oct 28, 2025 at 09:27:26PM -0300, Jason Gunthorpe wrote: > > On Sun, Oct 26, 2025 at 09:44:12PM -0700, Vivek Kasireddy wrote: > > > In a typical dma-buf use case, a dmabuf exporter makes its buffer > > > buffer available to an importer by mapping it using DMA APIs > > > such as dma_map_sgtable() or dma_map_resource(). However, this > > > is not desirable in some cases where the exporter and importer > > > are directly connected via a physical or virtual link (or > > > interconnect) and the importer can access the buffer without > > > having it DMA mapped. > > > > I think my explanation was not so clear, I spent a few hours and typed > > in what I was thinking about here: > > > > https://github.com/jgunthorpe/linux/commits/dmabuf_map_type > > > > I didn't type in the last patch for iommufd side, hopefully it is > > clear enough. Adding iov should follow the pattern of the "physical > > address list" patch. > > > > I think the use of EXPORT_SYMBOL_FOR_MODULES() to lock down the > > physical addres list mapping type to iommufd is clever and I'm hoping > > addresses Chrsitian's concerns about abuse. > > > > Single GPU drivers can easilly declare their own mapping type for > > their own private interconnect without needing to change the core > > code. > > > > This seems to be fairly straightforward and reasonably type safe.. > > It makes me wonder what am I supposed to do with my series now [1]? > How do you see submission plan now? > > [1] https://lore.kernel.org/all/cover.1760368250.git.leon@kernel.org/ IMHO that series needs the small tweaks and should go this merge window, ideally along with the iommufd half. I think this thread is a topic for the next cycle, I expect it will take some time to converge on the dmabuf core changes, and adapting your series is quite simple. Jason

1 day, 13 hours

Re: [RFC v2 0/8] dma-buf: Add support for mapping dmabufs via interconnects

by Jason Gunthorpe

On Sun, Oct 26, 2025 at 09:44:12PM -0700, Vivek Kasireddy wrote: > In a typical dma-buf use case, a dmabuf exporter makes its buffer > buffer available to an importer by mapping it using DMA APIs > such as dma_map_sgtable() or dma_map_resource(). However, this > is not desirable in some cases where the exporter and importer > are directly connected via a physical or virtual link (or > interconnect) and the importer can access the buffer without > having it DMA mapped. I think my explanation was not so clear, I spent a few hours and typed in what I was thinking about here: https://github.com/jgunthorpe/linux/commits/dmabuf_map_type I didn't type in the last patch for iommufd side, hopefully it is clear enough. Adding iov should follow the pattern of the "physical address list" patch. I think the use of EXPORT_SYMBOL_FOR_MODULES() to lock down the physical addres list mapping type to iommufd is clever and I'm hoping addresses Chrsitian's concerns about abuse. Single GPU drivers can easilly declare their own mapping type for their own private interconnect without needing to change the core code. This seems to be fairly straightforward and reasonably type safe.. What do you think? Jason

2 days, 1 hour

Re: [RFC 1/8] dma-buf: Add support for map/unmap APIs for interconnects

by Christian König

On 10/14/25 09:08, Vivek Kasireddy wrote: > For the map operation, the dma-buf core will create an xarray but > the exporter is expected to populate it with the interconnect > specific addresses. And, similarly for unmap, the exporter is > expected to cleanup the individual entries of the xarray. > > Cc: Jason Gunthorpe <jgg(a)nvidia.com> > Cc: Christian Koenig <christian.koenig(a)amd.com> > Cc: Sumit Semwal <sumit.semwal(a)linaro.org> > Cc: Thomas Hellström <thomas.hellstrom(a)linux.intel.com> > Cc: Simona Vetter <simona.vetter(a)ffwll.ch> > Signed-off-by: Vivek Kasireddy <vivek.kasireddy(a)intel.com> > --- > drivers/dma-buf/dma-buf.c | 68 ++++++++++++++++++++++++++++ > include/linux/dma-buf-interconnect.h | 29 ++++++++++++ > include/linux/dma-buf.h | 11 +++++ > 3 files changed, 108 insertions(+) > create mode 100644 include/linux/dma-buf-interconnect.h > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c > index 2bcf9ceca997..162642bd53e8 100644 > --- a/drivers/dma-buf/dma-buf.c > +++ b/drivers/dma-buf/dma-buf.c > @@ -1612,6 +1612,74 @@ void dma_buf_vunmap_unlocked(struct dma_buf *dmabuf, struct iosys_map *map) > } > EXPORT_SYMBOL_NS_GPL(dma_buf_vunmap_unlocked, "DMA_BUF"); > > +struct dma_buf_ranges * > +dma_buf_map_interconnect(struct dma_buf_attachment *attach) > +{ > + const struct dma_buf_interconnect_ops *ic_ops; > + struct dma_buf *dmabuf = attach->dmabuf; > + struct dma_buf_ranges *ranges; > + int ret; > + > + might_sleep(); > + > + if (WARN_ON(!attach || !attach->dmabuf)) > + return ERR_PTR(-EINVAL); > + > + if (!dma_buf_attachment_is_dynamic(attach)) > + return ERR_PTR(-EINVAL); > + > + if (!attach->allow_ic) > + return ERR_PTR(-EOPNOTSUPP); > + > + dma_resv_assert_held(attach->dmabuf->resv); > + > + ic_ops = dmabuf->ops->interconnect_ops; > + if (!ic_ops || !ic_ops->map_interconnect) > + return ERR_PTR(-EINVAL); > + > + ranges = kzalloc(sizeof(*ranges), GFP_KERNEL); > + if (!ranges) > + return ERR_PTR(-ENOMEM); > + > + xa_init(&ranges->ranges); > + ret = ic_ops->map_interconnect(attach, ranges); > + if (ret) > + goto err_free_ranges; > + > + return ranges; > + > +err_free_ranges: > + xa_destroy(&ranges->ranges); > + kfree(ranges); > + return ERR_PTR(ret); > +} > +EXPORT_SYMBOL_NS_GPL(dma_buf_map_interconnect, "DMA_BUF"); > + > +void dma_buf_unmap_interconnect(struct dma_buf_attachment *attach, > + struct dma_buf_ranges *ranges) > +{ > + const struct dma_buf_interconnect_ops *ic_ops; > + struct dma_buf *dmabuf = attach->dmabuf; > + > + if (WARN_ON(!attach || !attach->dmabuf || !ranges)) > + return; > + > + if (!attach->allow_ic) > + return; > + > + ic_ops = dmabuf->ops->interconnect_ops; > + if (!ic_ops || !ic_ops->unmap_interconnect) > + return; > + > + dma_resv_assert_held(attach->dmabuf->resv); > + > + ic_ops->unmap_interconnect(attach, ranges); > + > + xa_destroy(&ranges->ranges); > + kfree(ranges); > +} > +EXPORT_SYMBOL_NS_GPL(dma_buf_unmap_interconnect, "DMA_BUF"); > + > #ifdef CONFIG_DEBUG_FS > static int dma_buf_debug_show(struct seq_file *s, void *unused) > { > diff --git a/include/linux/dma-buf-interconnect.h b/include/linux/dma-buf-interconnect.h > new file mode 100644 > index 000000000000..17504dea9691 > --- /dev/null > +++ b/include/linux/dma-buf-interconnect.h > @@ -0,0 +1,29 @@ > +/* SPDX-License-Identifier: MIT */ > + > +#ifndef __DMA_BUF_INTERCONNECT_H__ > +#define __DMA_BUF_INTERCONNECT_H__ > + > +#include <linux/xarray.h> > + > +struct dma_buf_attachment; > + > +struct dma_buf_ranges { > + struct xarray ranges; > + unsigned int nranges; > +}; Hui? How is that supposed to work? Should the exporter fill in the xarray with values? That clearly needs more description. And IIRC xarray can only contain pointers because the lower bits are used for internal handling. Some kind of iterator like interface would be preferred where you have first and next callbacks. > + > +enum dma_buf_interconnect_type { > + DMA_BUF_INTERCONNECT_NONE = 0, Let's start with a DMA_BUF_DMA_ADDR type. > +}; > + > +struct dma_buf_interconnect { > + enum dma_buf_interconnect_type type; > +}; > + > +struct dma_buf_interconnect_ops { > + int (*map_interconnect)(struct dma_buf_attachment *attach, > + struct dma_buf_ranges *ranges); > + void (*unmap_interconnect)(struct dma_buf_attachment *attach, > + struct dma_buf_ranges *ranges); > +}; Please put those directly into the dma_buf_ops structure, I don't really see a value in separating them. Additional to that I'm not sure if the "interconnect" is a good naming, essentially we want to use the new mapping functions to replace the sg_table as well. Regards, Christian. > +#endif > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h > index d58e329ac0e7..db91c67c00d6 100644 > --- a/include/linux/dma-buf.h > +++ b/include/linux/dma-buf.h > @@ -23,6 +23,8 @@ > #include <linux/dma-fence.h> > #include <linux/wait.h> > > +#include <linux/dma-buf-interconnect.h> > + > struct device; > struct dma_buf; > struct dma_buf_attachment; > @@ -276,6 +278,8 @@ struct dma_buf_ops { > > int (*vmap)(struct dma_buf *dmabuf, struct iosys_map *map); > void (*vunmap)(struct dma_buf *dmabuf, struct iosys_map *map); > + > + const struct dma_buf_interconnect_ops *interconnect_ops; > }; > > /** > @@ -502,7 +506,9 @@ struct dma_buf_attachment { > struct device *dev; > struct list_head node; > bool peer2peer; > + bool allow_ic; > const struct dma_buf_attach_ops *importer_ops; > + struct dma_buf_interconnect interconnect; > void *importer_priv; > void *priv; > }; > @@ -589,6 +595,11 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *, > enum dma_data_direction); > void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *, > enum dma_data_direction); > + > +struct dma_buf_ranges *dma_buf_map_interconnect(struct dma_buf_attachment *); > +void dma_buf_unmap_interconnect(struct dma_buf_attachment *, > + struct dma_buf_ranges *); > + > void dma_buf_move_notify(struct dma_buf *dma_buf); > int dma_buf_begin_cpu_access(struct dma_buf *dma_buf, > enum dma_data_direction dir);

2 days, 10 hours

Re: [RFC v2 1/8] dma-buf: Add support for map/unmap APIs for interconnects

by Jason Gunthorpe

On Tue, Oct 28, 2025 at 05:39:39AM +0000, Kasireddy, Vivek wrote: > Hi Jason, > > > Subject: Re: [RFC v2 1/8] dma-buf: Add support for map/unmap APIs for > > interconnects > > > > On Sun, Oct 26, 2025 at 09:44:13PM -0700, Vivek Kasireddy wrote: > > > For the map operation, the dma-buf core will create an xarray but > > > the exporter needs to populate it with the interconnect specific > > > addresses. And, similarly for unmap, the exporter is expected to > > > cleanup the individual entries of the xarray. > > > > I don't think we should limit this to xarrays, nor do I think it is a > > great datastructure for what is usually needed here.. > One of the goals (as suggested by Christian) is to have a container that > can be used with an iterator. I thought Christian was suggesting to avoid the container and have some kind of iterator? > So, instead of creating a new data structure, > I figured using an xarray would make sense here. And, since the entries > of an xarray can be of any type, I think another advantage is that the > dma-buf core only needs to be aware of the xarray but the exporter can > use an interconnect specific type to populate the entries that the importer > would be aware of. It is excessively memory wasteful. > > I just posted the patches showing what iommufd needs, and it wants > > something like > > > > struct mapping { > > struct p2p_provider *provider; > > size_t nelms; > > struct phys_vec *phys; > > }; > > > > Which is not something that make sense as an xarray. > If we do not want to use an xarray, I guess we can try to generalize the > struct that holds the addresses and any additional info (such as provider). > Would any of the following look OK to you: I think just don't try to have a general struct, it is not required once we have interconnects. Each interconnect can define what makes sense for it. > struct dma_buf_ranges { > struct range *ranges; > unsigned int nranges; > void *ranges_data; > }; Like this is just pointless, it destroys type safety for no benifit. > > struct dma_buf_iov_interconnect_ops { > > struct dma_buf_interconnect_ops ic_ops; > > struct xx *(*map)(struct dma_buf_attachment *attach, > Do we want each specific interconnect to have its own return type for map? I think yes, then you have type safety and so on. The types should all be different. We need to get away from using dma_addr_t or phys_addr_t for something that is not in those address spaces. Jason

2 days, 13 hours

Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO regions

by Leon Romanovsky

On Mon, Oct 27, 2025 at 04:13:05PM -0700, David Matlack wrote: > On Mon, Oct 13, 2025 at 8:44 AM Leon Romanovsky <leon(a)kernel.org> wrote: > > > > From: Leon Romanovsky <leonro(a)nvidia.com> > > > > Add support for exporting PCI device MMIO regions through dma-buf, > > enabling safe sharing of non-struct page memory with controlled > > lifetime management. This allows RDMA and other subsystems to import > > dma-buf FDs and build them into memory regions for PCI P2P operations. > > > +/** > > + * Upon VFIO_DEVICE_FEATURE_GET create a dma_buf fd for the > > + * regions selected. > > + * > > + * open_flags are the typical flags passed to open(2), eg O_RDWR, O_CLOEXEC, > > + * etc. offset/length specify a slice of the region to create the dmabuf from. > > + * nr_ranges is the total number of (P2P DMA) ranges that comprise the dmabuf. > > + * > > + * Return: The fd number on success, -1 and errno is set on failure. > > + */ > > +#define VFIO_DEVICE_FEATURE_DMA_BUF 11 > > + > > +struct vfio_region_dma_range { > > + __u64 offset; > > + __u64 length; > > +}; > > + > > +struct vfio_device_feature_dma_buf { > > + __u32 region_index; > > + __u32 open_flags; > > + __u32 flags; > > + __u32 nr_ranges; > > + struct vfio_region_dma_range dma_ranges[]; > > +}; > > This uAPI would be a good candidate for a VFIO selftest. You can test > that it returns an error when it's supposed to, and a valid fd when > it's supposed to. And once the iommufd importer side is ready, we can > extend the test and verify that the fd can be mapped into iommufd. No problem, I'll add such test, but let's focus on making sure that this series is accepted first. Thanks

2 days, 13 hours

[PATCH 0/8] Initial DMABUF support for iommufd

by Jason Gunthorpe

This series is the start of adding full DMABUF support to iommufd. Currently it is limited to only work with VFIO's DMABUF exporter. It sits on top of Leon's series to add a DMABUF exporter to VFIO: https://lore.kernel.org/all/cover.1760368250.git.leon@kernel.org/ The existing IOMMU_IOAS_MAP_FILE is enhanced to detect DMABUF fd's, but otherwise works the same as it does today for a memfd. The user can select a slice of the FD to map into the ioas and if the underliyng alignment requirements are met it will be placed in the iommu_domain. Though limited, it is enough to allow a VMM like QEMU to connect MMIO BAR memory from VFIO to an iommu_domain controlled by iommufd. This is used for PCI Peer to Peer support in VMs, and is the last feature that the VFIO type 1 container has that iommufd couldn't do. The VFIO type1 version extracts raw PFNs from VMAs, which has no lifetime control and is a use-after-free security problem. Instead iommufd relies on revokable DMABUFs. Whenever VFIO thinks there should be no access to the MMIO it can shoot down the mapping in iommufd which will unmap it from the iommu_domain. There is no automatic remap, this is a safety protocol so the kernel doesn't get stuck. Userspace is expected to know it is doing something that will revoke the dmabuf and map/unmap it around the activity. Eg when QEMU goes to issue FLR it should do the map/unmap to iommufd. Since DMABUF is missing some key general features for this use case it relies on a "private interconnect" between VFIO and iommufd via the vfio_pci_dma_buf_iommufd_map() call. The call confirms the DMABUF has revoke semantics and delivers a phys_addr for the memory suitable for use with iommu_map(). Medium term there is a desire to expand the supported DMABUFs to include GPU drivers to support DPDK/SPDK type use cases so future series will work to add a general concept of revoke and a general negotiation of interconnect to remove vfio_pci_dma_buf_iommufd_map(). I also plan another series to modify iommufd's vfio_compat to transparently pull a dmabuf out of a VFIO VMA to emulate more of the uAPI of type1. The latest series for interconnect negotation to exchange a phys_addr is: https://lore.kernel.org/r/20251027044712.1676175-1-vivek.kasireddy@intel.com And the discussion for design of revoke is here: https://lore.kernel.org/dri-devel/20250114173103.GE5556@nvidia.com/ This is on github: https://github.com/jgunthorpe/linux/commits/iommufd_dmabuf The branch has various modifications to Leon's series I've suggested. Jason Gunthorpe (8): iommufd: Add DMABUF to iopt_pages iommufd: Do not map/unmap revoked DMABUFs iommufd: Allow a DMABUF to be revoked iommufd: Allow MMIO pages in a batch iommufd: Have pfn_reader process DMABUF iopt_pages iommufd: Have iopt_map_file_pages convert the fd to a file iommufd: Accept a DMABUF through IOMMU_IOAS_MAP_FILE iommufd/selftest: Add some tests for the dmabuf flow drivers/iommu/iommufd/io_pagetable.c | 74 +++- drivers/iommu/iommufd/io_pagetable.h | 53 ++- drivers/iommu/iommufd/ioas.c | 8 +- drivers/iommu/iommufd/iommufd_private.h | 13 +- drivers/iommu/iommufd/iommufd_test.h | 10 + drivers/iommu/iommufd/main.c | 10 + drivers/iommu/iommufd/pages.c | 407 ++++++++++++++++-- drivers/iommu/iommufd/selftest.c | 142 ++++++ tools/testing/selftests/iommu/iommufd.c | 43 ++ tools/testing/selftests/iommu/iommufd_utils.h | 44 ++ 10 files changed, 741 insertions(+), 63 deletions(-) base-commit: fc882154e421f82677925d33577226e776bb07a4 -- 2.43.0

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Linaro-mm-sig October 2025