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Re: [libvirt] [PATCH 4/6] qemu: Wire up disk model=virtio-{non-}transitional



On Tue, 2019-01-15 at 16:56 +0000, Daniel P. Berrangé wrote:
> On Sun, Jan 13, 2019 at 06:12:06PM -0500, Cole Robinson wrote:
> > Add new <disk> model values for virtio transitional devices. When
> > combined with bus='virtio':
> > 
> > * "virtio-transitional" maps to qemu "virtio-blk-pci-transitional"
> > * "virtio-non-transitional" maps to qemu "virtio-blk-pci-non-transitional"
> > 
> > Signed-off-by: Cole Robinson <crobinso redhat com>
> > ---
> >  src/qemu/qemu_command.c                       | 31 ++++++++++++++++++-
> >  src/qemu/qemu_domain_address.c                |  2 ++
> >  ...virtio-non-transitional.x86_64-latest.args |  7 +++--
> >  .../virtio-transitional.x86_64-latest.args    |  4 +--
> >  .../virtio-non-transitional.xml               | 10 ++++--
> >  5 files changed, 45 insertions(+), 9 deletions(-)
> > 
> > diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> > index 822d5f8669..ca6abea227 100644
> > --- a/src/qemu/qemu_command.c
> > +++ b/src/qemu/qemu_command.c
> > @@ -443,6 +443,33 @@ qemuBuildVirtioDevStr(virBufferPtr buf,
> >      return 0;
> >  }
> >  
> > +static int
> > +qemuBuildVirtioTransitional(virBufferPtr buf,
> > +                            const char *baseName,
> > +                            virDomainDeviceAddressType type,
> > +                            bool transitional,
> > +                            bool nontransitional)
> > +{
> > +    if (qemuBuildVirtioDevStr(buf, baseName, type) < 0)
> > +        return -1;
> > +
> > +    if (type != VIR_DOMAIN_DEVICE_ADDRESS_TYPE_PCI &&
> > +        (transitional || nontransitional)) {
> > +        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
> > +                       _("virtio transitional models are not supported "
> > +                         "for address type=%s"),
> > +                       virDomainDeviceAddressTypeToString(type));
> > +        return -1;
> > +    }
> > +
> > +    if (transitional) {
> > +        virBufferAddLit(buf, "-transitional");
> > +    } else if (nontransitional) {
> > +        virBufferAddLit(buf, "-non-transitional");
> > +    }
> > +    return 0;
> 
> So this only works on QEMU >= 4.0.0 - earlier versions will
> fail to start.
> 
> We can, however, make it work correctly with old QEMU.
> 
> A transitional device is 100% identical to the existing device
> types, so we can simply not add the "-transitional" suffix for
> old QEMU. The only difference is the way libvirt does PCI bus
> placement of the transitional device - we'd never use PCIe.
> 
> A non-transitional device is identical to the existing device
> types, but with  disable-legacy=true set.

Again, the relationship between existing and new devices is not
quite this straighforward because of the reasons I outlined in

  https://www.redhat.com/archives/libvir-list/2019-January/msg00514.html

But the idea of using disable-{legacy,modern} instead of the new
virtio-*-{non,}-transitional devices is one I had already suggested
in

  https://bugzilla.redhat.com/show_bug.cgi?id=1614127

so I'm obviously on board with it :)

> QEMU guarantees this compatibility of the different devices,
> but only for machine types < pc-i440fx-4.0.0 / pc-q35-4.0.0.
> So we should none the less make sure we use the modern device
> names for any QEMU which genuinely supports them.

As I already mentioned in the bug report linked above, I'm not
quite convinced that's the case, and I don't see why we wouldn't
just use the options and basically ignore the QEMU-level devices,
as the former approach would work on old QEMU releases as well as
recent ones with no drawback I can think of.

-- 
Andrea Bolognani / Red Hat / Virtualization


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