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

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Date:   Tue, 5 Dec 2017 18:56:05 +0100
From:   Radim Krcmar <rkrcmar@...hat.com>
To:     "Natarajan, Janakarajan" <Janakarajan.Natarajan@....com>
Cc:     Borislav Petkov <bp@...e.de>, Paolo Bonzini <pbonzini@...hat.com>,
        kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, Len Brown <len.brown@...el.com>,
        Kyle Huey <me@...ehuey.com>, Kan Liang <Kan.liang@...el.com>,
        Grzegorz Andrejczuk <grzegorz.andrejczuk@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH v2 3/4] Add support for AMD Core Perf Extension in guest

2017-12-01 13:30-0600, 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);
> > 
> > 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...
> 
> Paolo, Radim any suggestions about this. I feel that the number of counters
> initialized can be 6

This is the best solution with the current framework.

> and the subsequent code (kvm_pmu_refresh()) takes care of the number of
> counters to be used (4 or 6)
> based on the vcpu family.

Can't we look only at X86_FEATURE_PERFCTR_CORE and completely ignore the
AMD family?
Using the family would bring problems with compatiblity.

Thanks.