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Re: [libvirt] [PATCH 31/32] qemu: Use the 'device_id' property of SCSI disks to avoid regressing



On Tue, Jan 29, 2019 at 16:06:35 +0000, Daniel Berrange wrote:
> On Mon, Jan 28, 2019 at 05:19:00PM +0100, Peter Krempa wrote:
> > QEMU accidentally exposed the id of -drive (or same value as disk
> > serial, if provided) in one of the identifiers visible from the guest.
> > 
> > To avoid regression in case when -blockdev will be used we need to
> > always specify it ourselves.
> > 
> > Signed-off-by: Peter Krempa <pkrempa redhat com>
> > ---
> >  src/qemu/qemu_command.c                       | 22 +++++++++++++++++++
> >  .../controller-virtio-scsi.x86_64-latest.args | 20 ++++++++---------
> >  .../disk-cache.x86_64-latest.args             |  4 ++--
> >  .../disk-scsi-device-auto.x86_64-latest.args  |  3 ++-
> >  .../disk-scsi.x86_64-latest.args              | 16 ++++++++------
> >  .../disk-shared.x86_64-latest.args            |  5 +++--
> >  ...threads-virtio-scsi-pci.x86_64-latest.args |  4 ++--
> >  7 files changed, 50 insertions(+), 24 deletions(-)

[...]

> I rather wish there was a way we could avoid exposing the alias ID to every
> guest forever more.
> 
> QEMU could achieve that with machine type versioning, so that it only exposes
> the drive ID to guests with legacy machine types for sake of backport. We need
> to explicitly set this though, as with -blockdev QEMU can't do the right thing
> for legacy machine types as it lacks tie drive ID entirely. Once we set it,
> we enable it for non-legacy machine types too :-(  Annoyingly I don't see a
> way out of this mess such that libvirt only enables the back compat for
> existing guests.

Do you mean that for any new machine type we should not put the -drive
ID as the 'device_id' property? AFAIK qemu should now handle that
correctly by not creating any ID.

In that case we could tie -blockdev to the new machine type only and
thus will not need to add any code to format the backward compatibility
stuff.

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