Re: [PATCH RFCv1] virtio: Inherit max bounce buffer size from bus parent if possible

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 11, 2026 at 05:53:54PM +0100, Peter Maydell wrote:
> On Thu, 11 Jun 2026 at 17:42, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Jun 11, 2026 at 05:16:09PM +0100, Peter Maydell wrote:
> > > On Thu, 11 Jun 2026 at 16:57, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > Then I'd like to see an example where we have an actual good reason
> > > > to do execute arbitrary width accesses on device RAM when the
> > > > driver does not execute them, please.
> > >
> > > That's the case we started with, with this GPU where the
> > > virtio data structures are in this PCI BAR. We want the
> > > virtio backend to have the freedom to say "I'm just going
> > > to assume the ring buffer etc is in RAM, and I don't need to
> > > care about carefully ensuring that I only do word accesses".
> >
> > virtio backend does not need to assume anything. any spec
> > compliant guest guarantees it. any address given to
> > a virtio device is ram.
> 
> OK, but how do we tell if the mmap() PCI BAR we have is RAM
> or not ? Some of them are, some of them aren't...
> 
> -- PMM

We don't care? If guest asked a virtio device to access
memory then that memory better support accesses guest
requested, and on virtio that means any width.

-- 
MST