Re: [Qemu-devel] [PATCH RESEND v1 1/2] i386: Add Intel Processor Trace feature support

[Paolo Bonzini]

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.  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)

Others, currently only CPUID[EAX=14h,ECX=0].ECX[31] must match, there is
no way to emulate the "wrong" value.

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.

CPUID[EAX=14h,ECX=0].EAX is the maximum subleaf.  It should be (barring
guest bugs) okay to always present leaf 1.

Paolo