Re: [Qemu-devel] [PATCH] i386: extended the cpuid level when Intel PT is enabled

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Jan 28, 2019 at 06:06:56AM +0000, Kang, Luwei wrote:
> > > > > Intel Processor Trace required CPUID[0x14] but the cpuid level is
> > > > > 0xd when create a kvm guest with e.g. "-cpu qemu64,+intel-pt".
> > > > >
> > > > > Signed-off-by: Luwei Kang <luwei.kang@intel.com>
> > > > > ---
> > > > >  target/i386/cpu.c | 7 +++++++
> > > > >  1 file changed, 7 insertions(+)
> > > > >
> > > > > diff --git a/target/i386/cpu.c b/target/i386/cpu.c index
> > > > > 2f54125..da477b3 100644
> > > > > --- a/target/i386/cpu.c
> > > > > +++ b/target/i386/cpu.c
> > > > > @@ -5023,6 +5023,13 @@ static void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
> > > > >          x86_cpu_adjust_feat_level(cpu, FEAT_C000_0001_EDX);
> > > > >          x86_cpu_adjust_feat_level(cpu, FEAT_SVM);
> > > > >          x86_cpu_adjust_feat_level(cpu, FEAT_XSAVE);
> > > > > +
> > > > > +        /* Intel Processor Trace requires CPUID[0x14] */
> > > > > +        if ((env->features[FEAT_7_0_EBX] & CPUID_7_0_EBX_INTEL_PT) &&
> > > > > +             kvm_enabled()) {
> > > > > +            x86_cpu_adjust_level(cpu, &cpu->env.cpuid_min_level, 0x14);
> > > > > +        }
> > > >
> > > > This will require a new machine-type compatibility flag to enable the new behavior, so we don't change CPUID data under the guest feet
> > during live migration.
> > >
> > > Hi Eduardo,
> > >     Thanks for your reply. I have some question on your comments.
> > >     The cpuid level come from specific machine-type (e.g. qemu64, Skylake-Server) and they are all 0xd, but Intel PT required 0x14 so I
> > extend the cpuid level.
> > >     I don't fully understand what is the "require a new machine-type compatibility flag" mean, I need to add a new flag in each machine-
> > type?
> > >     I try to do live migration with "-cpu qemu64,+intel-pt" and "-cpu host" are all passed test. We didn't change the cpuid data during live
> > migration just initialize the cpuid data when create a new vcpu. Please correct me if anything wrong.
> > 
> > CPUID data is not sent as part of the migration stream (it is recreated on the migration destination), so if "-cpu qemu,+intel-pt"
> > results in different CPUID data, migration between QEMU 4.0 and
> > 3.1 will make CPUID level change during live migration.
> 
> Yes, CPUID data is not sent as part of migration stream. But
> when will the "-cpu qemu,+intel-pt" result different results? I
> think CPU type include some specific features and CPUID data of
> Intel PT are constant value, so I think the CPUID data should
> not change as well.

Before your patch, "-cpu qemu64,+intel-pt" has CPUID[0].EAX = 0xD,
and guest won't see intel-pt data in CPUID[0x14].

After your patch, "-cpu qemu64,+intel-pt" has CPUID[0].EAX = 0x14,
and guest will see intel-pt data in CPUID[0x14].

-- 
Eduardo