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

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Message-ID: <3e3cfa15-8a92-56e4-008e-4d4734efce21@amd.com>
Date:   Wed, 6 Dec 2017 14:19:11 -0600
From:   "Natarajan, Janakarajan" <Janakarajan.Natarajan@....com>
To:     Radim Krcmar <rkrcmar@...hat.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

On 12/5/2017 11:56 AM, Radim Krcmar wrote:
> 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.

Okay.

>
>> 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.

Yeah. We can just use the X86_FEATURE_PERFCTR_CORE flag as a check. I'll 
send a v3
with the changes.

Thanks.

>
> Thanks.