[libvirt] [PATCH 4/6] qemu: caps: Add capability for 'write-cache' parameter of disk frontends

[Peter Krempa]

On Wed, Apr 11, 2018 at 11:18:32 -0400, John Ferlan wrote:
> 
> 
> On 04/04/2018 04:13 AM, Peter Krempa wrote:
> > QEMU translates the cache mode of a disk internally into 3 flags.
> > 'write-cache' is a flag of the frontend while others are flag of the
> > backing storage. Add capability which will allow expressing it via the
> > frontend attribute.
> > 
> > Signed-off-by: Peter Krempa <pkrempa at redhat.com>
> > ---
> >  src/qemu/qemu_capabilities.c                       | 5 +++++
> >  src/qemu/qemu_capabilities.h                       | 1 +
> >  tests/qemucapabilitiesdata/caps_2.10.0.aarch64.xml | 1 +
> >  tests/qemucapabilitiesdata/caps_2.10.0.ppc64.xml   | 1 +
> >  tests/qemucapabilitiesdata/caps_2.10.0.s390x.xml   | 1 +
> >  tests/qemucapabilitiesdata/caps_2.10.0.x86_64.xml  | 1 +
> >  tests/qemucapabilitiesdata/caps_2.11.0.s390x.xml   | 1 +
> >  tests/qemucapabilitiesdata/caps_2.12.0.aarch64.xml | 1 +
> >  tests/qemucapabilitiesdata/caps_2.12.0.ppc64.xml   | 1 +
> >  tests/qemucapabilitiesdata/caps_2.12.0.s390x.xml   | 1 +
> >  tests/qemucapabilitiesdata/caps_2.12.0.x86_64.xml  | 1 +
> >  tests/qemucapabilitiesdata/caps_2.7.0.s390x.xml    | 1 +
> >  tests/qemucapabilitiesdata/caps_2.7.0.x86_64.xml   | 1 +
> >  tests/qemucapabilitiesdata/caps_2.8.0.s390x.xml    | 1 +
> >  tests/qemucapabilitiesdata/caps_2.8.0.x86_64.xml   | 1 +
> >  tests/qemucapabilitiesdata/caps_2.9.0.ppc64.xml    | 1 +
> >  tests/qemucapabilitiesdata/caps_2.9.0.s390x.xml    | 1 +
> >  tests/qemucapabilitiesdata/caps_2.9.0.x86_64.xml   | 1 +
> >  18 files changed, 22 insertions(+)
> > 
> 
> Will require merges with top of tree, but that's a trivial exercise.
> 
> Curious though - the testing patch 6 of this series would seem to imply
> that this feature is only available for at least QEMU 2.12 on x64_64;

I don't think that the DO_TEST_CAPS should be taken as an implication of
when the feature was introduced.

DO_TEST_CAPS is a better option for positive test cases than having to
explicitly name capabilities as with DO_TEST.

> however, the bit shows up starting in 2.7 on other arches, so since
> patch 5 only uses QEMU_CAPS_DISK_WRITE_CACHE to decide whether to use
> this or not, doesn't that imply earlier versions could use this?  or
> that the testing should be different?

No. DO_TEST_CAPS is not implying anything.
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