Re: [PATCH v3 1/4] virtio: skip legacy support check on machine types less than 5.1

[Stefano Garzarella]

On Fri, Sep 18, 2020 at 11:13:42AM +0200, Cornelia Huck wrote:
> On Fri, 18 Sep 2020 09:47:07 +0200
> Stefano Garzarella <sgarzare@redhat.com> wrote:
> 
> > Commit 9b3a35ec82 ("virtio: verify that legacy support is not accidentally
> > on") added a check that returns an error if legacy support is on, but the
> > device is not legacy.
> 
> s/is not legacy/does not support legacy/ ?
> 
> > 
> > Unfortunately some devices were wrongly declared legacy even if they
> 
> s/legacy/legacy capable/ ?
> 
> > were not (e.g vhost-vsock).
> > 
> > To avoid migration issues, we add a virtio-device property
> > (x-disable-legacy-check) to skip the legacy error, printing a warning
> > instaed, for machine types < 5.1.
> 
> s/instaed/instead/
> 

Much better, I'll fix them in v4.

> > 
> > Cc: qemu-stable@nongnu.org
> > Fixes: 9b3a35ec82 ("virtio: verify that legacy support is not accidentally 
> > on")
> > Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > Suggested-by: Cornelia Huck <cohuck@redhat.com>
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > ---
> > v3:
> >  - added virtio_legacy_check_disabled() helper
> >  - moved warning where error was returned [Cornelia]
> > v2:
> >  - fixed Cornelia's e-mail address
> > ---
> >  include/hw/virtio/virtio.h |  2 ++
> >  hw/core/machine.c          |  1 +
> >  hw/s390x/virtio-ccw.c      | 15 ++++++++++++---
> >  hw/virtio/virtio-pci.c     | 14 ++++++++++++--
> >  hw/virtio/virtio.c         |  7 +++++++
> >  5 files changed, 34 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > index 807280451b..f90cfb03e3 100644
> > --- a/include/hw/virtio/virtio.h
> > +++ b/include/hw/virtio/virtio.h
> > @@ -103,6 +103,7 @@ struct VirtIODevice
> >      bool use_started;
> >      bool started;
> >      bool start_on_kick; /* when virtio 1.0 feature has not been negotiated 
> > */
> > +    bool disable_legacy_check;
> >      VMChangeStateEntry *vmstate;
> >      char *bus_name;
> >      uint8_t device_endian;
> > @@ -396,5 +397,6 @@ static inline bool virtio_device_disabled(VirtIODevice 
> > *vdev)
> >  }
> >  
> >  bool virtio_legacy_allowed(VirtIODevice *vdev);
> > +bool virtio_legacy_check_disabled(VirtIODevice *vdev);
> >  
> >  #endif
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index ea26d61237..b686eab798 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -44,6 +44,7 @@ GlobalProperty hw_compat_5_0[] = {
> >      { "vmport", "x-signal-unsupported-cmd", "off" },
> >      { "vmport", "x-report-vmx-type", "off" },
> >      { "vmport", "x-cmds-v2", "off" },
> > +    { "virtio-device", "x-disable-legacy-check", "true" },
> >  };
> >  const size_t hw_compat_5_0_len = G_N_ELEMENTS(hw_compat_5_0);
> >  
> > diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> > index 8feb3451a0..97a01bc48a 100644
> > --- a/hw/s390x/virtio-ccw.c
> > +++ b/hw/s390x/virtio-ccw.c
> > @@ -1122,9 +1122,18 @@ static void virtio_ccw_device_plugged(DeviceState 
> > *d, Error **errp)
> >      }
> >  
> >      if (!virtio_ccw_rev_max(dev) && !virtio_legacy_allowed(vdev)) {
> > -        error_setg(errp, "Invalid value of property max_rev "
> > -                   "(is %d expected >= 1)", virtio_ccw_rev_max(dev));
> > -        return;
> > +        /*
> > +         * To avoid migration issues, we allow legacy mode when legacy
> > +         * check is disabled in the old machine types (< 5.1).
> > +         */
> > +        if (virtio_legacy_check_disabled(vdev)) {
> > +            warn_report("device is modern-only, but for backward "
> > +                        "compatibility legacy is allowed");
> 
> Hm...
> 
> "device requires revision >= 1, but for backward compatibility
> max_rev=0 is allowed"
> 
> seems more appropriate.

Me too, I forgot to update...

> 
> (I think I need to rethink the checking for ccw anyway.)
> 
> > +        } else {
> > +            error_setg(errp, "Invalid value of property max_rev "
> > +                       "(is %d expected >= 1)", virtio_ccw_rev_max(dev));
> > +            return;
> > +        }
> >      }
> >  
> >      if (virtio_get_num_queues(vdev) > VIRTIO_QUEUE_MAX) {
> 
> (...)
> 
> With the message for ccw changed,
> 
> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> 

Thanks,
Stefano