[Xen-devel] PVH cpuid feature flags

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Hello,

While doing some benchmarks on PV/PVH/PVHVM, I've realized that the
cpuid feature flags exposed to PVH guests are kind of strange, this is
the output of the feature flags as seen by an HVM domain:

Features=0x1783fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,MMX,FXSR,SSE,SSE2,HTT>
 Features2=0x81b82201<SSE3,SSSE3,CX16,SSE4.1,SSE4.2,x2APIC,POPCNT,TSCDLT,HV>
AMD Features=0x28100800<SYSCALL,NX,RDTSCP,LM>
AMD Features2=0x1<LAHF>

And this is what a PVH domain sees when running on the same hardware:

Features=0x1fc98b75<FPU,DE,TSC,MSR,PAE,CX8,APIC,SEP,CMOV,PAT,CLFLUSH,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT>
Features2=0x80982201<SSE3,SSSE3,CX16,SSE4.1,SSE4.2,POPCNT,HV>
AMD Features=0x20100800<SYSCALL,NX,LM>
AMD Features2=0x1<LAHF>

I would expect the feature flags to be quite similar between an HVM
domain and a PV domain (since they both run inside of a HVM container).
AFAIK, there's no reason to disable PSE, PGE, PSE36 and RDTSCP for PVH
guests. Also, is there any reason why PVH guests have the ACPI, SS and
CLFLUSH feature flags but not HVM?

Most (if not all) of this probably comes from the fact that we are
reporting the same feature flags as pure PV guests, but I see no reason
to do that for PVH guests, we should decide what's supported on PVH and
set the feature flags accordingly.

Roger.

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