Re: [PATCH v2 3/4] Add support for AMD Core Perf Extension in guest

[Natarajan, Janakarajan]

On 11/17/2017 5:44 AM, Borislav Petkov wrote:
> On Thu, Nov 16, 2017 at 12:00:11PM -0600, Natarajan, Janakarajan wrote:
> > Ah my apologies. So when the pmu is initialized the cpuid entries
> > aren't available then.
> So let's see:
>
> ... kvm_arch_vcpu_create() ->
> svm_create_vcpu() ->
> kvm_vcpu_init() ->
> kvm_arch_vcpu_init() ->
>
> <--- HERE
>
> kvm_pmu_init()
>
> But at HERE in kvm_arch_vcpu_init() right before kvm_pmu_init() we do already query
> cpuid:
>
> 	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);

cpuid_query_maxphyaddr first checks for for the 0x80000008 cpuid entry, 
and if it can't find it
returns a default value of 36.

When invoked in the kvm_arch_vcpu_init(), the default is being returned. 
The vcpu->arch.maxphyaddr
is later updated in the kvm_update_cpuid(), which in my case was setting 
the maxphyaddr to 40.
I believe the cpuid entry is not available when invoked in the init call 
and a default value is
being used as a placeholder until the entries are updated by qemu.

> so it's not like we don't know about cpuid leafs at that point. Which
> would mean that the code can be made to set the CPU family earlier,
> before kvm_pmu_init() runs so that you have the proper CPU family and
> thus have this thing properly designed.
>
> Maybe Paolo and Radim have a better suggestion here...