RE: [PATCH v1 Resend] target/i386: set the CPUID level to 0x14 on old machine-type

[Kang, Luwei]

> > The CPUID level need to be set to 0x14 manually on old machine-type if
> > Intel PT is enabled in guest. e.g. in Qemu 3.1 -machine pc-i440fx-3.1
> > -cpu qemu64,+intel-pt will be CPUID[0].EAX(level)=7 and
> > CPUID[7].EBX[25](intel-pt)=1.
> >
> > Some Intel PT capabilities are exposed by leaf 0x14 and the missing
> > capabilities will cause some MSRs access failed.
> > This patch add a warning message to inform the user to extend the
> > CPUID level.
> 
> Note that a warning is not an acceptable fix for a QEMU crash.
> We still need to fix the QEMU crash reported at:
> https://lore.kernel.org/qemu-devel/address@hidden/
> 
> 
> >
> > Suggested-by: Eduardo Habkost <address@hidden>
> > Signed-off-by: Luwei Kang <address@hidden>
> 
> The subject line says "v1", but this patch is different from the
> v1 you sent earlier.
> 
> If you are sending a different patch, please indicate it is a new version.  
> Please also
> indicate what changed between different patch versions, to help review.

Got it. I fix a code style problem in resending patch (remove the '\n').

ERROR: Error messages should not contain newlines
#36: FILE: target/i386/cpu.c:5448:
+                            "by \"-cpu ...,+intel-pt,level=0x14\"\n");
total: 1 errors, 0 warnings, 14 lines checked

> 
> > ---
> >  target/i386/cpu.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c index
> > a624163..f67c479 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -5440,8 +5440,12 @@ static void x86_cpu_expand_features(X86CPU
> > *cpu, Error **errp)
> >
> >          /* Intel Processor Trace requires CPUID[0x14] */
> >          if ((env->features[FEAT_7_0_EBX] & CPUID_7_0_EBX_INTEL_PT) &&
> > -             kvm_enabled() && cpu->intel_pt_auto_level) {
> 
> Not directly related to the warning: do you know why we have a
> kvm_enabled() check here?  It seems unnecessary.  We want CPUID level to be 
> correct
> for all accelerators.

Intel PT virtualization enabling in KVM guest need some hardware enhancement and
EPT must be enabled in KVM.  I think it can't work for e.g. tcg pure simulation 
accelerator.

> 
> > -            x86_cpu_adjust_level(cpu, &cpu->env.cpuid_min_level, 0x14);
> > +             kvm_enabled()) {
> > +            if (cpu->intel_pt_auto_level)
> > +                x86_cpu_adjust_level(cpu, &cpu->env.cpuid_min_level, 0x14);
> > +            else
> > +                warn_report("Intel PT need CPUID leaf 0x14, please set "
> > +                            "by \"-cpu ...,+intel-pt,level=0x14\"");
> 
> The warning shouldn't be triggered if level is already >= 0x14.
> 
> It is probably a good idea to mention that this happens only on
> pc-*-3.1 and older, as updating the machine-type is a better solution to the 
> problem
> than manually setting the "level"
> property.
> 
> This will print the warning multiple times if there are multiple VCPUs.  You 
> can use
> warn_report_once() to avoid that.

Got it. Will fix.

As you mentioned in this email " a warning is not an acceptable fix for a QEMU 
crash."
We can't change the configuration of the old machine type because it may break 
the
ABI compatibility. May I add more check on Intel PT, if CPUID[7].EBX[25] 
(intel-pt) = 1
and level is <0x14, mask off this feature? Or do you have any other suggestions?

Thanks,
Luwei Kang

> 
> >          }
> >
> >          /* CPU topology with multi-dies support requires CPUID[0x1F]
> > */
> > --
> > 1.8.3.1
> >
> 
> --
> Eduardo