linux-kernel - Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU state for Intel CPU

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Date: Sat, 13 Apr 2024 11:19:11 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>, Jim Mattson <jmattson@...gle.com>
Cc: Xiong Zhang <xiong.y.zhang@...ux.intel.com>, pbonzini@...hat.com,
 peterz@...radead.org, mizhang@...gle.com, kan.liang@...el.com,
 zhenyuw@...ux.intel.com, kvm@...r.kernel.org,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 zhiyuan.lv@...el.com, eranian@...gle.com, irogers@...gle.com,
 samantha.alt@...el.com, like.xu.linux@...il.com, chao.gao@...el.com
Subject: Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU
 state for Intel CPU


On 4/12/2024 7:31 AM, Sean Christopherson wrote:
> On Thu, Apr 11, 2024, Jim Mattson wrote:
>> On Thu, Apr 11, 2024 at 2:44 PM Sean Christopherson <seanjc@...gle.com> wrote:
>>>> +     /* Clear host global_ctrl and global_status MSR if non-zero. */
>>>> +     wrmsrl(MSR_CORE_PERF_GLOBAL_CTRL, 0);
>>> Why?  PERF_GLOBAL_CTRL will be auto-loaded at VM-Enter, why do it now?
>>>
>>>> +     rdmsrl(MSR_CORE_PERF_GLOBAL_STATUS, global_status);
>>>> +     if (global_status)
>>>> +             wrmsrl(MSR_CORE_PERF_GLOBAL_OVF_CTRL, global_status);
>>> This seems especially silly, isn't the full MSR being written below?  Or am I
>>> misunderstanding how these things work?
>> LOL! You expect CPU design to follow basic logic?!?
>>
>> Writing a 1 to a bit in IA32_PERF_GLOBAL_STATUS_SET sets the
>> corresponding bit in IA32_PERF_GLOBAL_STATUS to 1.
>>
>> Writing a 0 to a bit in to IA32_PERF_GLOBAL_STATUS_SET is a nop.
>>
>> To clear a bit in IA32_PERF_GLOBAL_STATUS, you need to write a 1 to
>> the corresponding bit in IA32_PERF_GLOBAL_STATUS_RESET (aka
>> IA32_PERF_GLOBAL_OVF_CTRL).
> If only C had a way to annotate what the code is doing. :-)
>
>>>> +     wrmsrl(MSR_CORE_PERF_GLOBAL_STATUS_SET, pmu->global_status);
> IIUC, that means this should be:
>
> 	if (pmu->global_status)
> 		wrmsrl(MSR_CORE_PERF_GLOBAL_STATUS_SET, pmu->global_status);
>
> or even better:
>
> 	toggle = pmu->global_status ^ global_status;
> 	if (global_status & toggle)
> 		wrmsrl(MSR_CORE_PERF_GLOBAL_OVF_CTRL, global_status & toggle);
> 	if (pmu->global_status & toggle)
> 		wrmsrl(MSR_CORE_PERF_GLOBAL_STATUS_SET, pmu->global_status & toggle);
Thanks, it looks better. Since PMU v4+, the MSR 
CORE_PERF_GLOBAL_OVF_CTRL is renamed to CORE_PERF_GLOBAL_STATUS_RESET 
with supporting more bits. CORE_PERF_GLOBAL_STATUS_RESET looks more 
easily understand just from name than CORE_PERF_GLOBAL_OVF_CTRL, I would 
prefer use this name in next version.