Re: [virtio-dev] Memory sharing device

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Gerd Hoffmann (kraxel@redhat.com) wrote:
>   Hi,
> 
> > > For communication between guest processes within the same VM I don't
> > > really see a need to involve the hypervisor ...
> > >
> > Right, once the host memory is set up we can rely on purely guest side stuff
> > map sub-regions of it.
> 
> Or just use guest ram ...
> 
> > > > Yes, also, other devices of the same VM.
> > >
> > > So why involve the hypervisor here?  The guest can handle that on its
> > > own.  Passing an image data buffer from the usb webcam to the intel gpu
> > > for display (on bare metal) isn't fundamentally different from passing a
> > > buffer from virtio-camera to virtio-gpu (in a VM).  Linux guests will
> > > use dma-bufs for that, other OSes probably something else.
> > 
> > That's true that it can be handled purely in the guest layers,
> > if there is an existing interface in the guest
> > to pass the proposed host memory id's / offsets / sizes
> > between them.
> 
> Note:  I think using a pci memory bar (aka host memory mapped into the
> guest) as backing storage for dma-bufs isn't going to work.

(Not knowing dma-bufs) but could you explain why?
Note in my spec the pci-bar isn't necessarily one chunk of host memory;
it's a chunk of host VMA into which multiple mmaps go.

Dave

> > However, for the proposed host memory sharing spec,
> > would there be a standard way to share the host memory across
> > different virtio devices without relying on Linux dmabufs?
> 
> I think with the current draft for each device (virtio-fs, virtio-gpu,
> ...) has its own device-specific memory, and there is no mechanism to
> exchange buffers between devices.
> 
> Stefan?
> 
> I'm also not convinced that explicitly avoiding dmabufs is a good idea
> here.  That would put virtio into its own universe and sharing buffers
> with non-virtio devices will not work.  Think about a intel vgpu as
> display device, or a usb camera attached to the guest using usb
> pass-through.
> 
> Experience shows that using virtualization-specific features /
> optimizations / short-cuts often turns out to have drawbacks in the long
> run, even if it looked like a good idea initially.  Just look at the
> mess we had with virtio-pci dma after iommu emulation landed in qemu.
> And this is only one example, we have more of this ...
> 
> cheers,
>   Gerd
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK