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. 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