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

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Message-ID: <20211005072006-mutt-send-email-mst@kernel.org>
Date:   Tue, 5 Oct 2021 07:20:20 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>, linux-s390@...r.kernel.org,
        markver@...ibm.com, Christian Borntraeger <borntraeger@...ibm.com>,
        qemu-devel@...gnu.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Xie Yongji <xieyongji@...edance.com>, stefanha@...hat.com,
        Raphael Norwitz <raphael.norwitz@...anix.com>
Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify

On Tue, Oct 05, 2021 at 01:13:31PM +0200, Cornelia Huck wrote:
> On Tue, Oct 05 2021, Halil Pasic <pasic@...ux.ibm.com> wrote:
> 
> > On Mon, 4 Oct 2021 05:07:13 -0400
> > "Michael S. Tsirkin" <mst@...hat.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.