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

[Paolo Bonzini]

On 28/09/20 14:42, Kang, Luwei wrote:
>> 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].

> 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