Re: [Xen-devel] PVH cpuid feature flags

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Tue, 21 Jan 2014 20:28:14 +0100
Roger Pau Monnà <roger.pau@xxxxxxxxxx> wrote:

> 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

Right, I already said I was working on xen patch to export CR4 and
other features. But, it will take couple days, as I'm debugging somethig
else right now. If you can't wait, go for it.

BTW, if you have time to work on PVH, can you pl help with migration?
Would be very much appreciated.... 

thanks
Mukesh


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