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Re: [libvirt] [PATCH 4/7] qemu: Add monitor APIs to fetch CPUID data from QEMU



On Wed, Jul 24, 2013 at 11:03:03AM +0100, Daniel P. Berrange wrote:
> On Tue, Jul 23, 2013 at 07:28:38PM +0200, Jiri Denemark wrote:
> > On Tue, Jul 23, 2013 at 17:32:42 +0100, Daniel Berrange wrote:
> > > On Tue, Jul 23, 2013 at 06:11:33PM +0200, Jiri Denemark wrote:
> > > > ---
> > > >  src/qemu/qemu_monitor.c                            |  21 +++
> > > >  src/qemu/qemu_monitor.h                            |   3 +
> > > >  src/qemu/qemu_monitor_json.c                       | 162 +++++++++++++++++++++
> > > >  src/qemu/qemu_monitor_json.h                       |   6 +
> > > >  tests/Makefile.am                                  |   1 +
> > > >  .../qemumonitorjson-getcpu-empty.data              |   2 +
> > > >  .../qemumonitorjson-getcpu-empty.json              |  46 ++++++
> > > >  .../qemumonitorjson-getcpu-filtered.data           |   4 +
> > > >  .../qemumonitorjson-getcpu-filtered.json           |  46 ++++++
> > > >  .../qemumonitorjson-getcpu-full.data               |   4 +
> > > >  .../qemumonitorjson-getcpu-full.json               |  46 ++++++
> > > >  .../qemumonitorjson-getcpu-host.data               |   5 +
> > > >  .../qemumonitorjson-getcpu-host.json               |  45 ++++++
> > > >  tests/qemumonitorjsontest.c                        |  74 ++++++++++
> > > >  14 files changed, 465 insertions(+)
> > > >  create mode 100644 tests/qemumonitorjsondata/qemumonitorjson-getcpu-empty.data
> > > >  create mode 100644 tests/qemumonitorjsondata/qemumonitorjson-getcpu-empty.json
> > > >  create mode 100644 tests/qemumonitorjsondata/qemumonitorjson-getcpu-filtered.data
> > > >  create mode 100644 tests/qemumonitorjsondata/qemumonitorjson-getcpu-filtered.json
> > > >  create mode 100644 tests/qemumonitorjsondata/qemumonitorjson-getcpu-full.data
> > > >  create mode 100644 tests/qemumonitorjsondata/qemumonitorjson-getcpu-full.json
> > > >  create mode 100644 tests/qemumonitorjsondata/qemumonitorjson-getcpu-host.data
> > > >  create mode 100644 tests/qemumonitorjsondata/qemumonitorjson-getcpu-host.json
> > > 
> > > ACK, though I believe the design of this monitor API is flawed
> > > because it requires you to re-launch QEMU with different accel
> > > args
> > 
> > Not really, this can be used in tcg too. It's just when we want to get
> > the data for "host" CPU, we need to enable kvm as tcg knows nothing
> > about that CPU. Which makes sense as kvm (the kernel module) influences
> > how the "host" CPU will look like.
> 
> Is there no ioctl() for the KVM module we can just invoke directly to
> discover what CPU flag filtering it will perform. Presumably QEMU is
> using some ioctl to discover this, so libvirt ought to be able to
> too.
> 

Yes, there is: GET_SUPPORTED_CPUID. But availability of some features
may depend on QEMU capabilities as well. On those cases libvirt may need
to combine the GET_SUPPORTED_CPUID results with what it knows about QEMU
capabilities. But this should work as long as we report and document
QEMU capabilities/options that affect CPU features very clearly.

That may be an appropriate way to go to, if you don't mind having
low-level KVM ioctl() code inside libvirt, that duplicates some QEMU
logic.

(But we still have the problem of querying/reporting CPU feature details
that depend on machine-type+CPU-model [that is not addressed by this
series yet]. See my previous message about it)

-- 
Eduardo


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