[PATCH v3 05/12] drm/ttm: Expose ttm_tt_unpopulate for driver use

Fri Nov 27 13:10:18 UTC 2020

Hey Daniel, just a ping on a bunch of questions i posted bellow.

Andtey
________________________________
From: Grodzovsky, Andrey <Andrey.Grodzovsky at amd.com>
Sent: 25 November 2020 14:34
To: Daniel Vetter <daniel at ffwll.ch>; Koenig, Christian <Christian.Koenig at amd.com>
Cc: robh at kernel.org <robh at kernel.org>; daniel.vetter at ffwll.ch <daniel.vetter at ffwll.ch>; dri-devel at lists.freedesktop.org <dri-devel at lists.freedesktop.org>; eric at anholt.net <eric at anholt.net>; ppaalanen at gmail.com <ppaalanen at gmail.com>; amd-gfx at lists.freedesktop.org <amd-gfx at lists.freedesktop.org>; gregkh at linuxfoundation.org <gregkh at linuxfoundation.org>; Deucher, Alexander <Alexander.Deucher at amd.com>; l.stach at pengutronix.de <l.stach at pengutronix.de>; Wentland, Harry <Harry.Wentland at amd.com>; yuq825 at gmail.com <yuq825 at gmail.com>
Subject: Re: [PATCH v3 05/12] drm/ttm: Expose ttm_tt_unpopulate for driver use

On 11/25/20 11:36 AM, Daniel Vetter wrote:
> On Wed, Nov 25, 2020 at 01:57:40PM +0100, Christian König wrote:
>> Am 25.11.20 um 11:40 schrieb Daniel Vetter:
>>> On Tue, Nov 24, 2020 at 05:44:07PM +0100, Christian König wrote:
>>>> Am 24.11.20 um 17:22 schrieb Andrey Grodzovsky:
>>>>> On 11/24/20 2:41 AM, Christian König wrote:
>>>>>> Am 23.11.20 um 22:08 schrieb Andrey Grodzovsky:
>>>>>>> On 11/23/20 3:41 PM, Christian König wrote:
>>>>>>>> Am 23.11.20 um 21:38 schrieb Andrey Grodzovsky:
>>>>>>>>> On 11/23/20 3:20 PM, Christian König wrote:
>>>>>>>>>> Am 23.11.20 um 21:05 schrieb Andrey Grodzovsky:
>>>>>>>>>>> On 11/25/20 5:42 AM, Christian König wrote:
>>>>>>>>>>>> Am 21.11.20 um 06:21 schrieb Andrey Grodzovsky:
>>>>>>>>>>>>> It's needed to drop iommu backed pages on device unplug
>>>>>>>>>>>>> before device's IOMMU group is released.
>>>>>>>>>>>> It would be cleaner if we could do the whole
>>>>>>>>>>>> handling in TTM. I also need to double check
>>>>>>>>>>>> what you are doing with this function.
>>>>>>>>>>>>
>>>>>>>>>>>> Christian.
>>>>>>>>>>> Check patch "drm/amdgpu: Register IOMMU topology
>>>>>>>>>>> notifier per device." to see
>>>>>>>>>>> how i use it. I don't see why this should go
>>>>>>>>>>> into TTM mid-layer - the stuff I do inside
>>>>>>>>>>> is vendor specific and also I don't think TTM is
>>>>>>>>>>> explicitly aware of IOMMU ?
>>>>>>>>>>> Do you mean you prefer the IOMMU notifier to be
>>>>>>>>>>> registered from within TTM
>>>>>>>>>>> and then use a hook to call into vendor specific handler ?
>>>>>>>>>> No, that is really vendor specific.
>>>>>>>>>>
>>>>>>>>>> What I meant is to have a function like
>>>>>>>>>> ttm_resource_manager_evict_all() which you only need
>>>>>>>>>> to call and all tt objects are unpopulated.
>>>>>>>>> So instead of this BO list i create and later iterate in
>>>>>>>>> amdgpu from the IOMMU patch you just want to do it
>>>>>>>>> within
>>>>>>>>> TTM with a single function ? Makes much more sense.
>>>>>>>> Yes, exactly.
>>>>>>>>
>>>>>>>> The list_empty() checks we have in TTM for the LRU are
>>>>>>>> actually not the best idea, we should now check the
>>>>>>>> pin_count instead. This way we could also have a list of the
>>>>>>>> pinned BOs in TTM.
>>>>>>> So from my IOMMU topology handler I will iterate the TTM LRU for
>>>>>>> the unpinned BOs and this new function for the pinned ones  ?
>>>>>>> It's probably a good idea to combine both iterations into this
>>>>>>> new function to cover all the BOs allocated on the device.
>>>>>> Yes, that's what I had in my mind as well.
>>>>>>
>>>>>>>> BTW: Have you thought about what happens when we unpopulate
>>>>>>>> a BO while we still try to use a kernel mapping for it? That
>>>>>>>> could have unforeseen consequences.
>>>>>>> Are you asking what happens to kmap or vmap style mapped CPU
>>>>>>> accesses once we drop all the DMA backing pages for a particular
>>>>>>> BO ? Because for user mappings
>>>>>>> (mmap) we took care of this with dummy page reroute but indeed
>>>>>>> nothing was done for in kernel CPU mappings.
>>>>>> Yes exactly that.
>>>>>>
>>>>>> In other words what happens if we free the ring buffer while the
>>>>>> kernel still writes to it?
>>>>>>
>>>>>> Christian.
>>>>> While we can't control user application accesses to the mapped buffers
>>>>> explicitly and hence we use page fault rerouting
>>>>> I am thinking that in this  case we may be able to sprinkle
>>>>> drm_dev_enter/exit in any such sensitive place were we might
>>>>> CPU access a DMA buffer from the kernel ?
>>>> Yes, I fear we are going to need that.
>>> Uh ... problem is that dma_buf_vmap are usually permanent things. Maybe we
>>> could stuff this into begin/end_cpu_access

Do you mean guarding with drm_dev_enter/exit in dma_buf_ops.begin/end_cpu_access
driver specific hook ?

>>> (but only for the kernel, so a
>>> bit tricky)?

Why only kernel ? Why is it a problem to do it if it comes from dma_buf_ioctl by
some user process ? And  if we do need this distinction I think we should be able to
differentiate by looking at current->mm (i.e. mm_struct) pointer being NULL for
kernel thread.

>> Oh very very good point! I haven't thought about DMA-buf mmaps in this
>> context yet.
>>
>>
>>> btw the other issue with dma-buf (and even worse with dma_fence) is
>>> refcounting of the underlying drm_device. I'd expect that all your
>>> callbacks go boom if the dma_buf outlives your drm_device. That part isn't
>>> yet solved in your series here.
>> Well thinking more about this, it seems to be a another really good argument
>> why mapping pages from DMA-bufs into application address space directly is a
>> very bad idea :)
>>
>> But yes, we essentially can't remove the device as long as there is a
>> DMA-buf with mappings. No idea how to clean that one up.
> drm_dev_get/put in drm_prime helpers should get us like 90% there I think.

What are the other 10% ?

>
> The even more worrying thing is random dma_fence attached to the dma_resv
> object. We could try to clean all of ours up, but they could have escaped
> already into some other driver. And since we're talking about egpu
> hotunplug, dma_fence escaping to the igpu is a pretty reasonable use-case.
>
> I have no how to fix that one :-/
> -Daniel

I assume you are referring to sync_file_create/sync_file_get_fence API  for
dma_fence export/import ?
So with DMA bufs we have the drm_gem_object as exporter specific private data
and so we can do drm_dev_get and put at the drm_gem_object layer to bind device
life cycle
to that of each GEM object but, we don't have such mid-layer for dma_fence which
could allow
us to increment device reference for each fence out there related to that device
- is my understanding correct ?

Andrey

Andrey

>> Christian.
>>
>>> -Daniel
>>>
>>>>> Things like CPU page table updates, ring buffer accesses and FW memcpy ?
>>>>> Is there other places ?
>>>> Puh, good question. I have no idea.
>>>>
>>>>> Another point is that at this point the driver shouldn't access any such
>>>>> buffers as we are at the process finishing the device.
>>>>> AFAIK there is no page fault mechanism for kernel mappings so I don't
>>>>> think there is anything else to do ?
>>>> Well there is a page fault handler for kernel mappings, but that one just
>>>> prints the stack trace into the system log and calls BUG(); :)
>>>>
>>>> Long story short we need to avoid any access to released pages after unplug.
>>>> No matter if it's from the kernel or userspace.
>>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>> Andrey
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