[libvirt] [PATCH 03/18] qemu: Support disk model=virtio-{non-}transitional

[Eduardo Habkost]

On Tue, Jan 22, 2019 at 12:45:59PM +0100, Andrea Bolognani wrote:
> On Tue, 2019-01-22 at 09:31 +0000, Daniel P. Berrangé wrote:
> > On Mon, Jan 21, 2019 at 05:41:47PM -0500, Cole Robinson wrote:
> > > On 01/18/2019 07:33 AM, Andrea Bolognani wrote:
> > > > On Thu, 2019-01-17 at 12:52 -0500, Cole Robinson wrote:
> > > > > +    if (has_tmodel) {
> > > > > +        if (virQEMUCapsGet(qemuCaps, tmodel_cap))
> > > > > +            virBufferAddLit(buf, "-transitional");
> > > > > +
> > > > > +        /* No error for if -transitional is not supported: our address
> > > > > +         * allocation will force the device into plain PCI bus, which
> > > > > +         * is functionally identical to standard 'virtio-XXX' behavior
> > > > > +         */
> > > > > +    } else if (has_ntmodel) {
> > > > > +        if (virQEMUCapsGet(qemuCaps, ntmodel_cap)) {
> > > > > +            virBufferAddLit(buf, "-non-transitional");
> > > > > +        } else if (virQEMUCapsGet(qemuCaps,
> > > > > +                                  QEMU_CAPS_VIRTIO_PCI_DISABLE_LEGACY)) {
> > > > > +            virBufferAddLit(buf, ",disable-legacy=on");
> > > > > +        } else {
> > > > > +            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
> > > > > +                           _("virtio non-transitional model not supported "
> > > > > +                             "for this qemu"));
> > > > > +            return -1;
> > > > > +        }
> > > > > +    }
> > > > 
> > > > Would it make sense to be more explicit here? Current versions of
> > > > QEMU default to disable-modern=off,disable-legacy=off for virtio-pci
> > > > devices plugged into conventional PCI slots, but unless I'm mistaken
> > > > that was not always the case, so it would perhaps be preferrable to
> > > > not rely on that behavior and always explicitly set both disable-*
> > > > options when the new devices are not available; if the options
> > > > themselves are not available, then we should error out.
> > > 
> > > I don't know enough to say, CCing ehabkost and danpb for more eyes
> > 
> > The QEMU code for the devices has
> > 
> >  - Original devs:  disable-modern=off disable-legacy=auto
> >  - Transitional devs: disable-modern=off disable-legacy=off
> >  - Non-transitional devs: disable-modern=off disable-legacy=on
> > 
> > IOW, in the case that -transitional is not available, we could
> > set disable-legacy=off. Provided that we always place -transitional
> > devices into PCI slots, never PCI-e slots, whether we set
> > disable-legacy=off or not doesn't have any effect. None the less it
> > would make sense to set it explicitly though as that would cause us
> > to catch bugs if we mistakenly had a -transitional dev in a PCI-e
> > slot.
> > 
> > I don't see a need to set disable-modern at all, since it is
> > defaulting to "off" in all cases.
> 
> Wasn't there some old QEMU release where disable-modern didn't
> default to off? I seem to remember that.

Yes, disable-modern is "on" on 2.6 and older machine-types.

> 
> More generally, I don't see a reason *not* to specify disable-modern
> along with disable-legacy. Why be implicit when you can very easily
> be explicit instead?

Agreed.

-- 
Eduardo