On Wed, 28 Nov 2018, Brian Starkey wrote:
Hi Liam,
On Tue, Nov 27, 2018 at 10:46:07PM -0800, Liam Mark wrote:
On Tue, 27 Nov 2018, Brian Starkey wrote:
Hi Liam,
On Mon, Nov 26, 2018 at 08:59:44PM -0800, Liam Mark wrote:
On Tue, 20 Nov 2018, Brian Starkey wrote:
Hi Liam,
I'm missing a bit of context here, but I did read the v1 thread. Please accept my apologies if I'm re-treading trodden ground.
I do know we're chasing nebulous ion "problems" on our end, which certainly seem to be related to what you're trying to fix here.
On Thu, Nov 01, 2018 at 03:15:06PM -0700, Liam Mark wrote:
Based on the suggestions from Laura I created a first draft for a change which will attempt to ensure that uncached mappings are only applied to ION memory who's cache lines have been cleaned. It does this by providing cached mappings (for uncached ION allocations) until the ION buffer is dma mapped and successfully cleaned, then it
drops
the userspace mappings and when pages are accessed they are faulted back in and uncached mappings are created.
If I understand right, there's no way to portably clean the cache of the kernel mapping before we map the pages into userspace. Is that right?
Yes, it isn't always possible to clean the caches for an uncached mapping because a device is required by the DMA APIs to do cache maintenance and there isn't necessarily a device available (dma_buf_attach may not yet have been called).
Alternatively, can we just make ion refuse to give userspace a non-cached mapping for pages which are mapped in the kernel as cached?
These pages will all be mapped as cached in the kernel for 64 bit (kernel logical addresses) so you would always be refusing to create a non-cached mapping.
And that might be the sane thing to do, no?
AFAIK there are still pages which aren't ever mapped as cached (e.g. dma_declare_coherent_memory(), anything under /reserved-memory marked as no-map). If those are exposed as an ion heap, then non-cached mappings would be fine, and permitted.
Sounds like you are suggesting using carveouts to support uncached?
No, I'm just saying that ion can't give out uncached _CPU_ mappings for pages which are already mapped on the CPU as cached.
Okay then I guess I am not clear on where you would get this memory which doesn't have a cached kernel mapping. It sounded like you wanted to define sections of memory in the DT as not mapped in the kernel and then hand this memory to dma_declare_coherent_memory (so that it can be managed) and then use an ION heap as the allocator. If the memory was defined this way it sounded a lot like a carveout. But I guess you have some thoughts on how this memory which doesn't have a kernel mapping can be made available for general use (for example available in buddy)?
Perhaps you were thinking of dynamically removing the kernel mappings before handing it out as uncached, but this would have a general system performance impact as this memory could come from anywhere so we would quickly lose our 1GB block mappings (and likely many of our 2MB block mappings as well).
We have many multimedia use cases which use very large amounts of uncached memory, uncached memory is used as a performance optimization because CPU access won't happen so it allows us to skip cache maintenance for all the dma map and dma unmap calls. To create carveouts large enough to support to support the worst case scenarios could result in very large carveouts.
Large carveouts like this would likely result in poor memory utilizations (since they are tuned for worst case) which would likely have significant performance impacts (more limited memory causes more frequent memory reclaim ect...).
Also estimating for worst case could be difficult since the amount of uncached memory could be app dependent. Unfortunately I don't think this would make for a very scalable solution.
Sure, I understand the desire not to use carveouts. I'm not suggesting carveouts are a viable alternative.
Would userspace using the dma-buf sync ioctl around its accesses do the "right thing" in that case?
I don't think so, the dma-buf sync ioctl require a device to peform cache maintenance, but as mentioned above a device may not be available.
If a device didn't attach yet, then no cache maintenance is necessary. The only thing accessing the memory is the CPU, via a cached mapping, which should work just fine. So far so good.
Unfortunately not. Scenario:
- Client allocates uncached memory.
- Client calls the DMA_BUF_IOCTL_SYNC IOCT IOCTL with flags
DMA_BUF_SYNC_START (but this doesn't do any cache maintenance since there isn't any device)
- Client mmap the memory (ION creates uncached mapping)
- Client reads from that uncached mapping
I think I maybe wasn't clear with my proposal. The sequence should be like this:
- Client allocates memory
- If this is from a region which the CPU has mapped as cached, then that's not "uncached" memory - it's cached memory - and you have to treat it as such.
- Client calls the DMA_BUF_IOCTL_SYNC IOCTL with flags DMA_BUF_SYNC_START (but this doesn't do any cache maintenance since there isn't any device)
- Client mmaps the memory
- ion creates a _cached_ mapping into the userspace process. ion *must not* create an uncached mapping.
- Client reads from that cached mapping
- It sees zeroes, as expected.
This proposal ensures that everyone will *always* see correct data if they use the DMA APIs properly (device accesses via dma_buf_{map,unmap}, CPU access via {begin,end}_cpu_access).
I am not sure I am properly understanding as this is what my V2 patch does, then when it gets an opportunity it allows the memory to be re-mapped as uncached.
Or are you perhaps suggesting that if the memory is allocated from a cached region then it always remains as cached, so only provide uncached if it was allocated from an uncached region? If so I view all memory available to the ION system heap for uncached allocations as having a cached mapping (since it is all part of the kernel logical mappigns), so I can't see how this would ever be able to support uncached allocations.
I guess once I understand how you will be providing memory to ION which isn't mapped as cached in the kernel, and therefore can be used to satisfy uncached ION allocations, this will make more sense to me.
Because memory has not been cleaned (we haven't had a device yet) the zeros that were written to this memory could still be in the cache (since they were written with a cached mapping), this means that the unprivilived userpace client is now potentially reading sensitive kernel data....
This is precisely why you can't just "pretend" that those pages are uncached. You can't have the same memory mapped with different attributes and get consistent behaviour.
If there are already attachments, then ion_dma_buf_begin_cpu_access() will sync for CPU access against all of the attached devices, and again the CPU should see the right thing.
In the other direction, ion_dma_buf_end_cpu_access() will sync for device access for all currently attached devices. If there's no attached devices yet, then there's nothing to do until there is (only thing accessing is CPU via a CPU-cached mapping).
When the first (or another) device attaches, then when it maps the buffer, the map_dma_buf callback should do whatever sync-ing is needed for that device.
I might be way off with my understanding of the various DMA APIs, but this is how I think they're meant to work.
Given that as you pointed out, the kernel does still have a cached mapping to these pages, trying to give the CPU a non-cached mapping of those same pages while preserving consistency seems fraught. Wouldn't it be better to make sure all CPU mappings are cached, and have CPU clients use the dma_buf_{begin,end}_cpu_access() hooks to get consistency where needed?
It is fraught, but unfortunately you can't rely on dma_buf_{begin,end}_cpu_access() to do cache maintenance as these calls require a device, and a device is not always available.
As above, if there's really no device, then no syncing is needed because only the CPU is accessing the buffer, and only ever via cached mappings.
Sure you can use cached mappings, but with cached memory to ensure cache coherency you would always need to do cache maintenance at dma map and dma unmap (since you can't rely on their being a device when dma_buf_{begin,end}_cpu_access() hooks are called).
As you've said below, you can't skip cache maintenance in the general case - the first time a device maps the buffer, you need to clean the cache to make sure the memset(0) is seen by the device.
Unfortunately if are only using cached mappings it isn't only the first time you dma map the buffer you need to do cache maintenance, you need to almost always do it because you don't know what CPU access happened (or will happen) without a device. Explained more below.
But with this cached memory you get poor performance because you are frequently doing cache mainteance uncessarily because there *could* be CPU access.
The reason we want to use uncached allocations, with uncached mappings, is to avoid all this uncessary cache maintenance.
OK I think this is the key - you don't actually care whether the mappings are non-cached, you just don't want to pay a sync penalty if the CPU never touched the buffer.
In that case, then to me the right thing to do is make ion use dma_map_sg_attrs(..., DMA_ATTR_SKIP_CPU_SYNC) in ion_map_dma_buf(), if it knows that the CPU hasn't touched the buffer (which it does - from {begin,end}_cpu_access).
Unfortunately that isn't the case we are trying to optimize for, we aren't trying to optimize for the case where CPU *never* touches the buffer we are trying to optimize for the case where the CPU may *rarely* touch the buffer.
If a client allocates cached memory the driver calling dma map and dma unmap has no way of knowing if at some pointe further down the pipeline there will be some userspace module which will attempt to do some kind of CPU access (example image library post processing). This userspace moduel will call the required DMA_BUF_IOCTL_SYNC IOCTLs, however there may no longer be a device attached, therefore these calls won't necessarily do the appropriate cache maintenance.
So what this means is that if a cached buffers is used you have to at least always to a cache invalidating when dma unmapping (from a device which isn't io-coherrent that did a write) otherwise there could be a CPU attempted to read that data using a cached mapping which could end up reading a stale cache line (for example acquired through speculative access).
This frequent uncessary cache maintenance adds a significant performance impact and that is why we use uncached memory because it allows us to skip all this cache maintenance. Basically your driver can't predict the future so it has to play it safe when cached ION buffers are involved.
Liam
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