[RFC v3 2/3] virtio: Introduce Vdmabuf driver

[Christian König]

Hi Vivek,

Am 10.02.21 um 05:47 schrieb Kasireddy, Vivek:
> Hi Gerd,
>
>> -----Original Message-----
>> From: Gerd Hoffmann <kraxel at redhat.com>
>> Sent: Tuesday, February 09, 2021 12:45 AM
>> To: Kasireddy, Vivek <vivek.kasireddy at intel.com>
>> Cc: Daniel Vetter <daniel at ffwll.ch>; virtualization at lists.linux-foundation.org; dri-
>> devel at lists.freedesktop.org; Vetter, Daniel <daniel.vetter at intel.com>;
>> daniel.vetter at ffwll.ch; Kim, Dongwon <dongwon.kim at intel.com>;
>> sumit.semwal at linaro.org; christian.koenig at amd.com; linux-media at vger.kernel.org
>> Subject: Re: [RFC v3 2/3] virtio: Introduce Vdmabuf driver
>>
>>    Hi,
>>
>>>>>> Nack, this doesn't work on dma-buf. And it'll blow up at runtime
>>>>>> when you enable the very recently merged CONFIG_DMABUF_DEBUG (would
>>>>>> be good to test with that, just to make sure).
>>> [Kasireddy, Vivek] Although, I have not tested it yet but it looks like this will
>>> throw a wrench in our solution as we use sg_next to iterate over all the struct page *
>>> and get their PFNs. I wonder if there is any other clean way to get the PFNs of all
>>> the pages associated with a dmabuf.
>> Well, there is no guarantee that dma-buf backing storage actually has
>> struct page ...
> [Kasireddy, Vivek] What if I do mmap() on the fd followed by mlock() or mmap()
> followed by get_user_pages()? If it still fails, would ioremapping the device memory
> and poking at the backing storage be an option? Or, if I bind the passthrough'd GPU device
> to vfio-pci and tap into the memory region associated with the device memory, can it be
> made to work?

get_user_pages() is not allowed on mmaped DMA-bufs in the first place.

Daniel is currently adding code to make sure that this is never ever used.

> And, I noticed that for PFNs that do not have valid struct page associated with it, KVM
> does a memremap() to access/map them. Is this an option?

No, even for system memory which has a valid struct page touching it 
when it is part of a DMA-buf is illegal since the reference count and 
mapping fields in struct page might be used for something different.

Keep in mind that struct page is a heavily overloaded structure for 
different use cases. You can't just use it for a different use case than 
what the owner of the page has intended it.

Regards,
Christian.

>
>>> [Kasireddy, Vivek] To exclude such cases, would it not be OK to limit the scope
>>> of this solution (Vdmabuf) to make it clear that the dma-buf has to live in Guest RAM?
>>> Or, are there any ways to pin the dma-buf pages in Guest RAM to make this
>>> solution work?
>> At that point it becomes (i915) driver-specific.  If you go that route
>> it doesn't look that useful to use dma-bufs in the first place ...
> [Kasireddy, Vivek] I prefer not to make this driver specific if possible.
>
>>> IIUC, Virtio GPU is used to present a virtual GPU to the Guest and all the rendering
>>> commands are captured and forwarded to the Host GPU via Virtio.
>> You don't have to use the rendering pipeline.  You can let the i915 gpu
>> render into a dma-buf shared with virtio-gpu, then use virtio-gpu only for
>> buffer sharing with the host.
> [Kasireddy, Vivek] Is this the most viable path forward? I am not sure how complex or
> feasible it would be but I'll look into it.
> Also, not using the rendering capabilities of virtio-gpu and turning it into a sharing only
> device means there would be a giant mode switch with a lot of if() conditions sprinkled
> across. Are you OK with that?
>
> Thanks,
> Vivek
>> take care,
>>    Gerd