Re: [virtio-comment] Re: [PATCH 09/11] transport-pci: Describe PCI MMR dev config registers

[Jason Wang <jasowang@redhat.com>]

On Mon, Apr 10, 2023 at 2:40âPM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Apr 10, 2023 at 02:20:16PM +0800, Jason Wang wrote:
> > On Mon, Apr 10, 2023 at 2:15âPM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Mon, Apr 10, 2023 at 09:33:32AM +0800, Jason Wang wrote:
> > > > This is fine for vDPA but not for virtio if the design can only work
> > > > for some specific setups (OSes/archs).
> > > >
> > > > Thanks
> > >
> > > Well virtio legacy has a long history of documenting existing hacks :)
> >
> > Exactly, so the legacy behaviour is not (or can't be) defined by the
> > spec but the codes.
>
> I mean driver behaviour derives from the code but we do document it in
> the spec to help people build devices.
>
>
> > > But yes, VIRTIO_F_ORDER_PLATFORM has to be documented.
> > > And we have to decide what to do about ACCESS_PLATFORM since
> > > there's a security problem if device allows not acking it.
> > > Two options:
> > > - relax the rules a bit and say device will assume ACCESS_PLATFORM
> > >   is acked anyway
> >
> > This will break legacy drivers which assume physical addresses.
>
> not that they are not already broken.

I may miss something, the whole point is to allow legacy drivers to
run otherwise a modern device is sufficient?

>
> > > - a new flag that is insecure (so useful for sec but useless for dpdk) but optional
> >
> > This looks like a new "hack" for the legacy hacks.
>
> it's not just for legacy.

We have the ACCESS_PLATFORM feature bit, what is the useage for this new flag?

>
> > And what about ORDER_PLATFORM, I don't think we can modify legacy drivers...
> >
> > Thanks
>
> You play some tricks with shadow VQ I guess.

Do we really want to add a new feature in the virtio spec that can
only work with the datapath mediation?

Thanks

>
> --
> MST
>