[virglrenderer-devel] Proposals for virtio-gpu and virglrenderer

[Tomeu Vizoso]

On 04/27/2018 03:05 AM, Dave Airlie wrote:
>> I've been working on adding accelerated virtio-gpu support to crosvm as
>> well as adding support for sending dmabufs across virtio-wayland for
>> improved host-side compositor performance. Along the way, I've encountered
>> a few features that I would like to discuss to make things work better for
>> crosvm, and, I believe, other users of virtio-gpu/virglrenderer.
> 
> Hey,
> 
> I'm just back from a few weeks off so I might need help on these as I'm
> not fully back up to speed.
>>
>> 1) Add mmap support to virtio-gpu
>> 2) Add API to import buffers to virglrenderer
>> 3) Harden virglrenderer using memory safe language
>>
>> Elaboration for each feature below:
>>
>> 1) Currently, the only way to fill a resource in virtio-gpu is either via a
>> transfer from the backing memory to the resource, or to submit commands
>> that render to the resource. This is not optimal for software rendering or
>> texture upload situations because of the additional copy this transfer
>> necessitates. I propose that an additional virtio-gpu command, perhaps
>> gated by a virtio feature bit, be made that allows the guest to request
>> mapping of a resource directly into guest physical address space. With this
>> virtio command, requests to mmap a virgio-gpu resource by the guest's user
>> space applications, perhaps through a DRM ioctl on the buffer object handle
>> or an mmap on the dmabuf/prime FD, could be fulfilled without additional
>> copy operations to flush data that was written.
> 
> How does this deal with tiling/unmappable resources? Most of the resources
> we allocate end up in tile VRAM hopefully, this may not even be mappable.
> When you use the GL APIs to map it we end up getting a transfer not a direct
> map so you get a copy there. If we bypass GL and go straight to exporting
> the buffer, we just get a tiled buffer which isn't that useful.
> 
> Lets then assume you only want to map linear strided resources, you envisage
> exporting a precreated resource to a dma-buf, and somehow mmaping
> the pages behind that resource into the guest memory space.
> 
> Now the rules for what you can map where in guest VM are a bit hazy for me,
> maybe someone can expend more on how we can do this, do we need a PCI
> BAR to have these things appear in or can they end up in random guest pages?
> Gerd?

Gerd will be able to give you a more authoritative answer, but last time 
we talked about it the only safe way of doing it is with PCI BARs.

KVM_SET_USER_MEMORY_REGION is what crosvm uses atm, but Gerd warned that 
the guest kernel has no knowledge of the mapping, as it's not involved 
during the process.

Regards,

Tomeu