RE: [PATCH v1 1/3] i386: Remove the limitation of IP payloads for Intel PT

[Kang, Luwei]

> >> No, if a feature cannot be emulated, that means it cannot be enabled
> >> unless it matches the host.  That's generally not a problem since
> >> Intel PT is usually used only with "-cpu host".
> >>
> > The limitation of LIP in qemu will mask off the Intel PT in KVM guest
> > even with "-cpu host". e.g. This bit will be set in SnowRidge HW and
> > later.
> 
> I agree that QEMU would have to learn about LIP.  Unlike this patch, however,
> x86_cpu_filter_features would have to fail if host LIP != guest LIP.  That is,
> something like
> 
>            (ecx_0 & INTEL_PT_IP_LIP)) !=
>               (env->features[INTEL_PT_ECX_0] & INTEL_PT_IP_LIP)
> 
> where "intel-pt-lip" would be a feature in env->features[INTEL_PT_ECX_0].

Got it. Thanks.

Luwei Kang

> 
> > How about "-cpu cpu_model, +intel-pt" use case? The current value of
> > Intel PT CPUID is a constant. Can we make the ICX CPUID as basic
> > inforation(LIP is 0) and using "+intel-pt-lip" to make Intel PT work
> > on the CPU which LIP is 1 on the host? As you mentioned before, Intel
> > PT cannot be enabled in guest unless it matches the host.
> 
> Yes, this would work.
> 
> Paolo