Re: [PATCH] vhost: accept VIRTIO_F_ORDER_PLATFORM as a valid SVQ feature

[Eugenio Perez Martin]

On Thu, Mar 2, 2023 at 12:43 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Mar 02, 2023 at 12:30:52PM +0100, Eugenio Perez Martin wrote:
> > > You need to pass this to guest. My point is that there is no reason to
> > > get it from the kernel driver. QEMU can figure out whether the flag is
> > > needed itself.
> > >
> >
> > Ok, I can see now how the HW device does not have all the knowledge to
> > offer this flag or not. But I'm not sure how qemu can know either.
> >
> > If qemu opens /dev/vhost-vdpa-N, how can it know it? It has no way to
> > tell if the device is sw or hw as far as I know. Am I missing
> > something?
> >
> > Thanks!
>
> This is what I said earlier.  You can safely assume vdpa needs this
> flag. Only exception is vduse and we don't care about performance there.
>

Ok now I get your point, thanks for explaining.

But I'm missing why it is wrong to start using it properly from the
kernel. I didn't test vDPA in non x86 / PCI, but if it does not work
because of the lack of this feature flag the right fix would be to
offer it, not to start assuming it in qemu, isn't it?

I can see how "assume VIRTIO_F_ORDER_PLATFORM from qemu" may need code
comments and extra explanations, but to start offering it properly
from the device is expected somehow.

Thanks!