linux-kernel - Re: [RFC PATCH 1/1] virtio: write back features before verify

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Date:   Wed, 6 Oct 2021 08:15:08 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        Jason Wang <jasowang@...hat.com>,
        Xie Yongji <xieyongji@...edance.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, markver@...ibm.com,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org, qemu-devel@...gnu.org
Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify

On Wed, Oct 06, 2021 at 12:13:14PM +0200, Cornelia Huck wrote:
> On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Mon, Oct 04, 2021 at 05:50:44PM +0200, Cornelia Huck wrote:
> >> On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> >> 
> >> > On Mon, Oct 04, 2021 at 04:33:21PM +0200, Cornelia Huck wrote:
> >> >> On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> >> >> 
> >> >> > On Mon, Oct 04, 2021 at 02:19:55PM +0200, Cornelia Huck wrote:
> >> >> >> 
> >> >> >> [cc:qemu-devel]
> >> >> >> 
> >> >> >> On Sat, Oct 02 2021, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> >> >> >> 
> >> >> >> > ok so that's a QEMU bug. Any virtio 1.0 and up
> >> >> >> > compatible device must use LE.
> >> >> >> > It can also present a legacy config space where the
> >> >> >> > endian depends on the guest.
> >> >> >> 
> >> >> >> So, how is the virtio core supposed to determine this? A
> >> >> >> transport-specific callback?
> >> >> >
> >> >> > I'd say a field in VirtIODevice is easiest.
> >> >> 
> >> >> The transport needs to set this as soon as it has figured out whether
> >> >> we're using legacy or not.
> >> >
> >> > Basically on each device config access?
> >> 
> >> Prior to the first one, I think. It should not change again, should it?
> >
> > Well yes but we never prohibited someone from poking at both ..
> > Doing it on each access means we don't have state to migrate.
> 
> Yes; if it isn't too high overhead, that's probably the safest way to
> handle it.
> 
> >
> >> >
> >> >> I guess we also need to fence off any
> >> >> accesses respectively error out the device if the driver tries any
> >> >> read/write operations that would depend on that knowledge?
> >> >> 
> >> >> And using a field in VirtIODevice would probably need some care when
> >> >> migrating. Hm...
> >> >
> >> > It's just a shorthand to minimize changes. No need to migrate I think.
> >> 
> >> If we migrate in from an older QEMU, we don't know whether we are
> >> dealing with legacy or not, until feature negotiation is already
> >> done... don't we have to ask the transport?
> >
> > Right but the only thing that can happen is config access.
> 
> Checking on each config space access would be enough then.
> 
> > Well and for legacy a kick I guess.
> 
> I think any driver that does something that is not config space access,
> status access, or feature bit handling without VERSION_1 being set is
> neccessarily legacy? Does that really need special handling?

Likely not, I just wanted to be exact.

-- 
MST