linux-kernel - Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest

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Message-ID: <9fa07da4-8ef4-f1f2-72a0-5d747e283912@linux.intel.com>
Date:   Fri, 22 Jan 2021 13:30:20 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Kan Liang <kan.liang@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, eranian@...gle.com,
        kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Andi Kleen <andi@...stfloor.org>, wei.w.wang@...el.com,
        luwei.kang@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and
 inject it to guest

On 2021/1/16 1:42, Sean Christopherson wrote:
> On Fri, Jan 15, 2021, Xu, Like wrote:
>> On 2021/1/15 2:55, Sean Christopherson wrote:
>>> On Mon, Jan 04, 2021, Like Xu wrote:
>>>> +	 * Note: KVM disables the co-existence of guest PEBS and host PEBS.
>>> By "KVM", do you mean KVM's loading of the MSRs provided by intel_guest_get_msrs()?
>>> Because the PMU should really be the entity that controls guest vs. host.  KVM
>>> should just be a dumb pipe that handles the mechanics of how values are context
>>> switch.
>>
>> The intel_guest_get_msrs() and atomic_switch_perf_msrs()
>> will work together to disable the co-existence of guest PEBS and host PEBS:
>>
>> https://lore.kernel.org/kvm/961e6135-ff6d-86d1-3b7b-a1846ad0e4c4@intel.com/
>>
>> +
>>
>> static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>> ...
>>      if (nr_msrs > 2 && (msrs[1].guest & msrs[0].guest)) {
>>          msrs[2].guest = pmu->ds_area;
>>          if (nr_msrs > 3)
>>              msrs[3].guest = pmu->pebs_data_cfg;
>>      }
>>
>>     for (i = 0; i < nr_msrs; i++)
>> ...
> 
> Yeah, that's exactly what I'm complaining about.  Splitting the logic for
> determining the guest values is unnecessarily confusing, and as evidenced by the
> PEBS_ENABLE bug, potentially fragile.  Perf should have full knowledge and
> control of what values are loaded for the guest.  And, the above indexing magic
> is nigh impossible to follow and _super_ fragile.

Thanks for pointing this out.

> 
> If we change .guest_get_msrs() to take a struct kvm_pmu pointer, then it can
> generate the full set of guest values by grabbing ds_area and pebs_data_cfg.
> Alternatively, .guest_get_msrs() could take the desired guest MSR values
> directly (ds_area and pebs_data_cfg), but kvm_pmu is vendor agnostic, so I don't
> see any reason to not just pass the pointer.

Hi Peter,

What do you think of us passing a "struct kvm_pmu" pointer (defined in 
arch/x86/include/asm/kvm_host.h) to guest_get_msrs(int *nr) ?

---
thx,likexu