Linux Virtualization - Oct 2021 - [RFC PATCH 1/1] virtio: write back features before verify

On Tue, Oct 05 2021, Halil Pasic <pasic at linux.ibm.com> wrote:
> On Mon, 4 Oct 2021 05:07:13 -0400
> "Michael S. Tsirkin" <mst at redhat.com> wrote:
>> Well we established that we can know. Here's an alternative
explanation:
>
>
> I thin we established how this should be in the future, where a transport
> specific mechanism is used to decide are we operating in legacy mode or
> in modern mode. But with the current QEMU reality, I don't think so.
> Namely currently the switch native-endian config -> little endian config
> happens when the VERSION_1 is negotiated, which may happen whenever
> the VERSION_1 bit is changed, or only when FEATURES_OK is set
> (vhost-user).
>
> This is consistent with device should detect a legacy driver by checking
> for VERSION_1, which is what the spec currently says.
>
> So for transitional we start out with native-endian config. For modern
> only the config is always LE.
>
> The guest can distinguish between a legacy only device and a modern
> capable device after the revision negotiation. A legacy device would
> reject the CCW.
>
> But both a transitional device and a modern only device would accept
> a revision > 0. So the guest does not know for ccw.
Well, for pci I think the driver knows that it is using either legacy or
modern, no?

And for ccw, the driver knows at that point in time which revision it
negotiated, so it should know that a revision > 0 will use LE (and the
device will obviously know that as well.)

Or am I misunderstanding what you're getting at?

On Tue, Oct 05, 2021 at 01:13:31PM +0200, Cornelia Huck
wrote:
> On Tue, Oct 05 2021, Halil Pasic <pasic at linux.ibm.com> wrote:
> 
> > On Mon, 4 Oct 2021 05:07:13 -0400
> > "Michael S. Tsirkin" <mst at redhat.com> wrote:
> >> Well we established that we can know. Here's an alternative
explanation:
> >
> >
> > I thin we established how this should be in the future, where a
transport
> > specific mechanism is used to decide are we operating in legacy mode
or
> > in modern mode. But with the current QEMU reality, I don't think
so.
> > Namely currently the switch native-endian config -> little endian
config
> > happens when the VERSION_1 is negotiated, which may happen whenever
> > the VERSION_1 bit is changed, or only when FEATURES_OK is set
> > (vhost-user).
> >
> > This is consistent with device should detect a legacy driver by
checking
> > for VERSION_1, which is what the spec currently says.
> >
> > So for transitional we start out with native-endian config. For modern
> > only the config is always LE.
> >
> > The guest can distinguish between a legacy only device and a modern
> > capable device after the revision negotiation. A legacy device would
> > reject the CCW.
> >
> > But both a transitional device and a modern only device would accept
> > a revision > 0. So the guest does not know for ccw.
> 
> Well, for pci I think the driver knows that it is using either legacy or
> modern, no?
> 
> And for ccw, the driver knows at that point in time which revision it
> negotiated, so it should know that a revision > 0 will use LE (and the
> device will obviously know that as well.)
> 
> Or am I misunderstanding what you're getting at?
Exactly what I'm saying.

On Tue, 05 Oct 2021 13:13:31 +0200
Cornelia Huck <cohuck at redhat.com> wrote:
> On Tue, Oct 05 2021, Halil Pasic <pasic at linux.ibm.com> wrote:
> 
> > On Mon, 4 Oct 2021 05:07:13 -0400
> > "Michael S. Tsirkin" <mst at redhat.com> wrote:  
> >> Well we established that we can know. Here's an alternative
explanation:
> >
> >
> > I thin we established how this should be in the future, where a
transport
> > specific mechanism is used to decide are we operating in legacy mode
or
> > in modern mode. But with the current QEMU reality, I don't think
so.
> > Namely currently the switch native-endian config -> little endian
config
> > happens when the VERSION_1 is negotiated, which may happen whenever
> > the VERSION_1 bit is changed, or only when FEATURES_OK is set
> > (vhost-user).
> >
> > This is consistent with device should detect a legacy driver by
checking
> > for VERSION_1, which is what the spec currently says.
> >
> > So for transitional we start out with native-endian config. For modern
> > only the config is always LE.
> >
> > The guest can distinguish between a legacy only device and a modern
> > capable device after the revision negotiation. A legacy device would
> > reject the CCW.
> >
> > But both a transitional device and a modern only device would accept
> > a revision > 0. So the guest does not know for ccw.  
> 
> Well, for pci I think the driver knows that it is using either legacy or
> modern, no?
It is mighty complicated. virtio-blk-pci-non-transitional and 
virtio-net-pci-non-transitional will give you BE, but 
virtio-crypto-pci, which is also non-transitional will get you LE,
before VERSION_1 is set (becausevirtio-crypto uses stl_le_p()). That is
fact.

The deal is that virtio-blk and virtion-net was written with
transitional in mind, and config code is the same for transitional and
non-transitional.

That is how things are now. With the QEMU changes things will be simpler.
> 
> And for ccw, the driver knows at that point in time which revision it
> negotiated, so it should know that a revision > 0 will use LE (and the
> device will obviously know that as well.)
With the future changes in QEMU, yes. Without these changes no. Without
these changes we get BE when the guest code things it is going to get
LE. That is what causes the regression.

The commit message for this patch is written from the perspective of
right now, and not from the perspective of future changes.

Or can you hack up a guest patch that looks at the revision, figures out
what endiannes is the early config access in, and does the right thing?

I don't think so. I tried to explain why that is impossible. Because
that would be preferable to messing with the the device and introducing
another exit. 
> 
> Or am I misunderstanding what you're getting at?
> 
Probably. I'm talking about pre- "do transport specific legacy
detection
in the device instead of looking at VERSION_1" you are probably talking
about the post-state. If we had this new behavior for all relevant
hypervisors then we wouldn't need to do a thing in the guest. The current
code would work like charm.

Does that answer your question?

Regards,
Halil