Re: [PATCH 1/2] scsi-disk: Advertise FUA support by default

[Kevin Wolf <kwolf@redhat.com>]

Am 04.03.2025 um 17:15 hat Daniel P. Berrangé geschrieben:
> On Tue, Mar 04, 2025 at 03:52:31PM +0000, Alberto Faria wrote:
> > FUA emulation code is already is place.
> > 
> > Signed-off-by: Alberto Faria <afaria@redhat.com>
> > ---
> >  hw/scsi/scsi-disk.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
> > index e7f738b484..8cf50845ab 100644
> > --- a/hw/scsi/scsi-disk.c
> > +++ b/hw/scsi/scsi-disk.c
> > @@ -3212,7 +3212,7 @@ static const Property scsi_hd_properties[] = {
> >      DEFINE_PROP_BIT("removable", SCSIDiskState, features,
> >                      SCSI_DISK_F_REMOVABLE, false),
> >      DEFINE_PROP_BIT("dpofua", SCSIDiskState, features,
> > -                    SCSI_DISK_F_DPOFUA, false),
> > +                    SCSI_DISK_F_DPOFUA, true),
> 
> Should this come with machine type compat to prevent us advertizing FUA
> to existing machine type versions ?

To give a little more background to Alberto: When upgrading QEMU, you
don't want the guest-visible machine to change. You already don't really
want hardware changes while the VM is down (Windows might require
reactivation etc.), but we support live migration between old and new
versions (often in both directions) as long as you use the same machine
type, and no guest expects that the hardware changes while it's running.

So we can only change guest-visible features in new machine types. What
you set here is fine for all new machine types, but you'll have to set
it back to false in hw_compat_9_2 in hw/core/machine.c so that all older
machine types still get the old default.

Kevin