Am 13.12.21 um 12:18 schrieb Shunsuke Mie:
2021年12月10日(金) 22:29 Christian König christian.koenig@amd.com:
Am 10.12.21 um 14:26 schrieb Jason Gunthorpe:
On Fri, Dec 10, 2021 at 01:47:37PM +0100, Christian König wrote:
Am 10.12.21 um 13:42 schrieb Jason Gunthorpe:
On Fri, Dec 10, 2021 at 08:29:24PM +0900, Shunsuke Mie wrote:
Hi Jason, Thank you for replying.
2021年12月8日(水) 2:14 Jason Gunthorpe jgg@ziepe.ca: > On Fri, Dec 03, 2021 at 12:51:44PM +0900, Shunsuke Mie wrote: >> Hi maintainers, >> >> Could you please review this patch series? > Why is it RFC? > > I'm confused why this is useful? > > This can't do copy from MMIO memory, so it shouldn't be compatible > with things like Gaudi - does something prevent this? I think if an export of the dma-buf supports vmap, CPU is able to access the mmio memory.
Is it wrong? If this is wrong, there is no advantages this changes..
I don't know what the dmabuf folks did, but yes, it is wrong.
IOMEM must be touched using only special accessors, some platforms crash if you don't do this. Even x86 will crash if you touch it with something like an XMM optimized memcpy.
Christian? If the vmap succeeds what rules must the caller use to access the memory?
See dma-buf-map.h and especially struct dma_buf_map.
MMIO memory is perfectly supported here and actually the most common case.
Okay that looks sane, but this rxe RFC seems to ignore this completely. It stuffs the vaddr directly into a umem which goes to all manner of places in the driver.
??
Well, yes that can go boom pretty quickly.
Sorry, I was wrong. The dma_buf_map treats both iomem and vaddr region, but this RFC only supports vaddr. Advantage of the partial support is we can use the vaddr dma-buf in RXE without changing a rxe data copy implementation.
Well that is most likely not a good idea.
For example buffers for GPU drivers can be placed in both MMIO memory and system memory.
If you don't want to provoke random failures you *MUST* be able to handle both if you want to use this.
Regards, Christian.
An example of a dma-buf pointing to a vaddr is some gpu drivers use RAM for VRAM and we can get dma-buf for the region that indicates vaddr regions. Specifically, the gpu driver using gpu/drm/drm_gem_cma_helper.c is one such example.
Not sure what they want to use this for.
I'd like to use RDMA with RXE for that memory region.
Best, Shunsuke
Christian.
Jason