about mmap dma-buf and sync

Thomas Hellstrom thellstrom at vmware.com
Fri Aug 21 15:00:21 PDT 2015

On 08/21/2015 06:00 PM, Jerome Glisse wrote:
> On Fri, Aug 21, 2015 at 04:15:53PM +0200, Thomas Hellstrom wrote:
>> On 08/21/2015 03:32 PM, Jerome Glisse wrote:
>>> On Fri, Aug 21, 2015 at 07:25:07AM +0200, Thomas Hellstrom wrote:
>>>> On 08/20/2015 10:34 PM, Jerome Glisse wrote:
>>>>> On Thu, Aug 20, 2015 at 09:39:12PM +0200, Thomas Hellstrom wrote:
>>>>>> On 08/20/2015 04:53 PM, Jerome Glisse wrote:
>>>>>>> On Thu, Aug 20, 2015 at 08:48:23AM +0200, Thomas Hellstrom wrote:
>>>>>>>> Hi, Tiago!
>>>>>>>> On 08/20/2015 12:33 AM, Tiago Vignatti wrote:
>>>>>>>>> Hey Thomas, you haven't answered my email about making SYNC_* mandatory:
>>>>>>>>> https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.freedesktop.org_archives_dri-2Ddevel_2015-2DAugust_088376.html&d=BQIDAw&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=vpukPkBtpoNQp2IUKuFviOmPNYWVKmen3Jeeu55zmEA&m=EX3w8PM79N5qLeXyogeSPN9J5pIvBV6IKrhBjzwJDEM&s=S3NqF0kPNtBKQ3-WiffL2T9NFK96XBp56vkb3ujf-io&e= 
>>>>>>>> Hmm, for some reason it doesn't show up in my mail app, but I found it
>>>>>>>> in the archives. An attempt to explain the situation from the vmwgfx
>>>>>>>> perspective.
>>>>>>>> The fact that the interface is generic means that people will start
>>>>>>>> using it for the zero-copy case. There has been a couple of more or less
>>>>>>>> hackish attempts to do this before, and if it's a _driver_ interface we
>>>>>>>> don't need to be that careful but if it is a _generic_ interface we need
>>>>>>>> to be very careful to make it fit *all* the hardware out there and that
>>>>>>>> we make all potential users use the interface in a way that conforms
>>>>>>>> with the interface specification.
>>>>>>>> What will happen otherwise is that apps written for coherent fast
>>>>>>>> hardware might, for example, ignore calling the SYNC api, just because
>>>>>>>> the app writer only cared about his own hardware on which the app works
>>>>>>>> fine. That would fail miserably if the same app was run on incoherent
>>>>>>>> hardware, or the incoherent hardware driver maintainers would be forced
>>>>>>>> to base an implementation on page-faults which would be very slow.
>>>>>>>> So assume the following use case: An app updates a 10x10 area using the
>>>>>>>> CPU on a 1600x1200 dma-buf, and it will then use the dma-buf for
>>>>>>>> texturing. On some hardware the dma-buf might be tiled in a very
>>>>>>>> specific way, on vmwgfx the dma-buf is a GPU buffer on the host, only
>>>>>>>> accessible using DMA. On vmwgfx the SYNC operation must carry out a
>>>>>>>> 10x10 DMA from the host GPU buffer to a guest CPU buffer before the CPU
>>>>>>>> write and a DMA back again after the write, before GPU usage. On the
>>>>>>>> tiled architecture the SYNC operation must untile before CPU access and
>>>>>>>> probably tile again before GPU access.
>>>>>>>> If we now have a one-dimensional SYNC api, in this particular case we'd
>>>>>>>> either need to sync a far too large area (1600x10) or call SYNC 10 times
>>>>>>>> before writing, and then again after writing. If the app forgot to call
>>>>>>>> SYNC we must error.
>>>>>>>> So to summarize, the fact that the interface is generic IMO means:
>>>>>>>> 1) Any user must be able to make valid assumptions about the internal
>>>>>>>> format of the dma-buf. (untiled, color format, stride etc.)
>>>>>>>> 2) Any user *must* call SYNC before and after CPU access. On coherent
>>>>>>>> architectures, the SYNC is a NULL operation anyway, and that should be
>>>>>>>> documented somewhere so that maintainers of drivers of uncoherent
>>>>>>>> architectures have somewhere to point their fingers.
>>>>>>>> 3) Driver-specific implementations must be allowed to error (segfault)
>>>>>>>> if SYNC has not been used.
>>>>>>> I think here you are too lax, the common code must segfault or
>>>>>>> error badly if SYNC has not been use in all cases even on cache
>>>>>>> coherent arch. The device driver sync callback can still be a
>>>>>>> no operation. But i think that we need to insist strongly on a
>>>>>>> proper sync call being made (and we should forbid empty area
>>>>>>> sync call). This would be the only way to make sure userspace
>>>>>>> behave properly as otherwise we endup in the situation you were
>>>>>>> describing above, where the app is design on a cache coherent
>>>>>>> arch and works fine there but broke in subtle way on non cache
>>>>>>> coherent arch and app developer is clueless of why.
>>>>>>> I just do not trust userspace.
>>>>>> I agree and ideally i'd want it this way as well. The question is, is it
>>>>>> possible? Can this be done without a fault() handler in the generic
>>>>>> kernel code?
>>>>>>>> 4) The use-case stated above clearly shows the benefit of a
>>>>>>>> 2-dimensional sync interface (we want to sync the 10x10 region), but
>>>>>>>> what if someone in the future wants to use this interface for a 3D
>>>>>>>> texture? Will a 2D sync suffice? Can we make the SYNC interface
>>>>>>>> extendable in a way that an enum sync_type member defines the layout of
>>>>>>>> the argument, and initially we implement only 1d, 2d sync, leaving 3d
>>>>>>>> for the future?
>>>>>>>> Also, I agree there is probably no good way to generically implement an
>>>>>>>> error if SYNC has not been called. That needs to be left as an option to
>>>>>>>> drivers.
>>>>>>> I think there is, just forbid any further use of the dma buffer, mark
>>>>>>> it as invalid and printk a big error. Userspace app developer will
>>>>>>> quickly see that something is wrong and looking at kernel log should
>>>>>>> explain why.
>>>>>> The problem is if someone calls mmap() and then decides to not access
>>>>>> the buffer before munmap() or GPU usage. How do we recognize that case
>>>>>> and separate it from a CPU access occured outside a sync? We could, as
>>>>>> mentioned above have a fault() handler in the generic kernel code and
>>>>>> make sure drivers don't populate PTEs until the first fault(). Another
>>>>>> option would be to scan all PTEs for an "accessed" flag, but I'm not
>>>>>> even sure all CPU architectures have such a flag?
>>>>>> But there might be a simpler solution that I have overlooked?
>>>>> All arch have a dirty flag, so you could check that, as all we
>>>>> really care about is CPU write access. So all you need is clearing
>>>>> the dirty bit after each successfull GPU command stream ioctl
>>>>> and checking that no dirty bit is set in a region not cover by
>>>>> a flush(). Note that the clear and check can happen in the same
>>>>> function as part of buffer validation for the GPU command stream.
>>>> Actually, I do think we care about reading as well, since reading
>>>> without flushing anything written by the GPU will
>>>> yield garbage on a non-coherent architecture. Instead we might check for
>>>> the PAGE_ACCESSED bit.
>>> I do not think the read access after GPU matter. If people think it does
>>> then it is simple, at buffer validation in buffer mapping we invalidate
>>> CPU page table mapping and the prepare access ioctl populate them after
>>> checking that GPU is flush.
>>>> So the full version of this would keep track of all synced regions and
>>>> at buffer validation time, or unmap time error if there
>>>> is a page completely outside the union of all synced regions in any VMA
>>>> belonging to the dma-buf address space, then unmark all accessed PTEs
>>>> and invalidate TLBs accordingly, typically causing a global TLB flush.
>>>> Yeah, we could probably put together a helper function that does this,
>>>> but it will be expensive to run and the whole point of this API is to be
>>>> able to improve performance.
>>> No need for such complexity, the sync ioctl would go over the page table
>>> and clear dirty bit for all page in range that is synchronized. So once
>>> you get to the command ioctl you know that all dirty bit must be clear.
>>> Note this is only a page table walk in all case. I do not consider this
>>> to be that expensive.
>> The expensive part is the TLB flushing after the PTE update, which
>> typically needs to be performed on all cores. Although without a
>> benchmark I'm not sure how expensive. It might be that it doesn't
>> matter. After all, the vmwgfx- and I believe the qxl driver both use a
>> similar approach for fbdev user-space access, but that has never been
>> cosidered performance-critical...
> There is no need to flush the TLB ! At least not for the write case.
> Clearing the dirty bit is cache coherent. TLB are about caching page
> table directory tree. To convince yourself look at page_mkclean_one()
> in mm/rmap.c which, by the way, is almost what we want (remove the mmu
> notifier part and the write protection part and force real CPU cache
> flush if needed base on device flag).

Hmm? Last time I visited this code, page_mkclean_one() was eventually
calling flush_tlb_page() through ptep_clear_flush_notify(). I was under
the impression that you always have to flush the TLB when you clear a
PTE status bit, but typically not when you set it. Are you saying that
that's not the case?
The question as I see it is rather whether you want to flush TLB on a
per-PTE basis or do it as a global flush after a batched PTE status bit


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