Re: [libvirt] [PATCH RESEND v1 1/2] i386: Add Intel Processor Trace feature support
> On 18/01/2018 14:24, Eduardo Habkost wrote:
> > However, if there's a simple way to make it possible to migrate
> > between hosts with different CPUID[14h] data, it would be even
> > better. With the current KVM intel-pt implementation, what happens
> > if the CPUID[14h] data seen by the guest doesn't match exactly the
> > CPUID[14h] leaves from the host?
>
> Some bits in there can be treated as CPU features (e.g. EBX bit 0 "CR3
> filtering support"). Probably we should handle these in KVM right now.
> KVM needs to compute a mask of valid 1 bits for IA32_RTIT_CTL based on
> CPUID, and apply it when the MSR is written.
Does this mean QEMU can't set CPUID values that won't match the host with the
existing implementation, or this won't matter for
well-behaved guests that don't try to set reserved bits on the MSRs?
> It also needs to
> whitelist bits like we do for other feature words. These include:
>
> - CPUID[EAX=14h,ECX=0].EBX
>
> - CPUID[EAX=14h,ECX=0].ECX except bit 31
>
> - CPUID[EAX=14h,ECX=1].EAX bits 16:31 (if
> CPUID[EAX=14h,ECX=0].EBX[3]=1)
>
> - CPUID[EAX=14h,ECX=1].EBX (if CPUID[EAX=14h,ECX=0].EBX[1]=1)
What do you mean by whitelist?
>
> Others, currently only CPUID[EAX=14h,ECX=0].ECX[31] must match, there
> is no way to emulate the "wrong" value.
In this case we could make it configurable but require the host and guest value to always
match.
This might be an obstacle to enabling intel-pt by default (because it could make VMs not
migratable to newer hosts), but may allow
the feature to be configured in a predictable way.
>
> Others, currently only CPUID[EAX=14h,ECX=1].EAX[2:0] are numeric
> values, and it's possible to emulate a lower value than the one in the
processor.
This could be handled by QEMU. There's no requirement that all GET_SUPPORTED_CPUID
values should be validated by simple bit
masking.
So, we can get a lower value on the numeric of CPUID[EAX=14h,ECX=1].EAX[2:0] between
different host. How about other bits of CPUID[14] ? Can we do like this as well?
Thanks,
Luwei Kang
>
> CPUID[EAX=14h,ECX=0].EAX is the maximum subleaf. It should be
> (barring guest bugs) okay to always present leaf 1.
>
> Paolo
--
Eduardo