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Date: Mon, 8 May 2017 12:56:14 +0200
From: Kashyap Chamarthy <kchamart@redhat.com>
Message-ID: <20170508105614.4zigymw3khttiarj@eukaryote>
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Subject: Re: [Qemu-devel] [PATCH 4/4] x86: Support feature=force on the
 command-line
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To: Eduardo Habkost <ehabkost@redhat.com>
Cc: qemu-devel@nongnu.org, Michael Roth <mdroth@linux.vnet.ibm.com>, Alexander Graf <agraf@suse.de>, Markus Armbruster <armbru@redhat.com>, Igor Mammedov <imammedo@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>

On Fri, May 05, 2017 at 02:59:24PM -0300, Eduardo Habkost wrote:
> On Thu, May 04, 2017 at 12:16:38PM +0200, Kashyap Chamarthy wrote:
> > On Tue, May 02, 2017 at 05:31:15PM -0300, Eduardo Habkost wrote:
> > 
> > Hi,
> > 
> > Yet to try this series, a small question in-line.
> > 
> > > Introduce a new CPUFeatureSetting QAPI data type, and use it to support
> > > feature=force on -cpu.
> > > 
> > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> [...]
> > > @@ -336,5 +340,13 @@ int main(int argc, char **argv)
> > >                       "-machine accel=kvm:tcg -cpu max,mmx=off",
> > >                       1, 0, "EDX", 23, false);
> > >  
> > > +    {
> > > +    FeatureTestArgs *a;
> > > +    a = add_feature_test("x86/cpuid/features/monitor-force",
> > > +                         "-machine accel=kvm:tcg -cpu 486,monitor=force",
> > 
> > Following your above test example, should the 'force' boolean also work
> > (understood: only for testing / debugging) as below, for a recognized
> > CPUID flag, taking the example of 'INVPCID'?
> > 
> >   $ qemu-system-x86_64 [...] -cpu Haswell-noTSX,invpcid=force [...]
> 
> This would be valid syntax and would enable invpcid on CPUID,
> yes. But on most cases this means you will get a broken VCPU
> because the host won't virtualize the feature properly (if it
> did, the feature would be reported as available in
> GET_SUPPORTED_CPUID and "feature=on" would already work).

Thanks for the clarification.  (/me today learnt that
GET_SUPPORTED_CPUID is a KVM ioctl(2).


-- 
/kashyap