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 6:06âPM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Apr 10, 2023 at 03:20:52PM +0800, Jason Wang wrote:
> > 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?
>
> yes and if legacy drivers don't work in a given setup then we
> should not worry.
>
> > >
> > > > > - 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?
>
>
> ACCESS_PLATFORM is also a security boundary. so devices must fail
> negotiation if it's not there. this new one won't be.
>
>
> > >
> > > > 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
>
> As long as a feature is useful and can't be supported otherwise
> we are out of options.

Probably not? Is it as simple as relaxing this:

"Transitional devices MUST expose the Legacy Interface in I/O space in BAR0."

To allow memory space.

This works for both software and hardware devices (I had a handy
hardware that supports legacy virtio drivers in this way).

Thanks

> Keeping field practice things out of the
> spec helps no one.
>
> > >
> > > --
> > > MST
> > >
>