linux-kernel - Re: [PATCH V7 03/18] perf/x86/intel: Handle guest PEBS overflow PMI for KVM guest

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Date:   Thu, 8 Jul 2021 21:39:50 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     pbonzini@...hat.com, bp@...en8.de, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, weijiang.yang@...el.com,
        kan.liang@...ux.intel.com, ak@...ux.intel.com,
        wei.w.wang@...el.com, eranian@...gle.com, liuxiangdong5@...wei.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        like.xu.linux@...il.com, Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH V7 03/18] perf/x86/intel: Handle guest PEBS overflow PMI
 for KVM guest



On 7/2/2021 7:28 PM, Peter Zijlstra wrote:
> On Tue, Jun 22, 2021 at 05:42:51PM +0800, Zhu Lingshan wrote:
>> +DECLARE_STATIC_CALL(x86_guest_state, *(perf_guest_cbs->state));
>> +
>> +/*
>> + * We may be running with guest PEBS events created by KVM, and the
>> + * PEBS records are logged into the guest's DS and invisible to host.
>> + *
>> + * In the case of guest PEBS overflow, we only trigger a fake event
>> + * to emulate the PEBS overflow PMI for guest PBES counters in KVM.
>> + * The guest will then vm-entry and check the guest DS area to read
>> + * the guest PEBS records.
>> + *
>> + * The contents and other behavior of the guest event do not matter.
>> + */
>> +static void x86_pmu_handle_guest_pebs(struct pt_regs *regs,
>> +				      struct perf_sample_data *data)
>> +{
>> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>> +	u64 guest_pebs_idxs = cpuc->pebs_enabled & ~cpuc->intel_ctrl_host_mask;
>> +	struct perf_event *event = NULL;
>> +	unsigned int guest = 0;
>> +	int bit;
>> +
>> +	if (!x86_pmu.pebs_vmx || !x86_pmu.pebs_active ||
>> +	    !(cpuc->pebs_enabled & ~cpuc->intel_ctrl_host_mask))
>> +		return;
>> +
>> +	guest = static_call(x86_guest_state)();
>> +	if (!(guest & PERF_GUEST_ACTIVE))
>> +		return;
> I think you've got the branches the wrong way around here; nobody runs a
> VM so this branch will get you out without a load.
>
> Only if you're one of those daft people running a VM, are you interested
> in any of the other conditions that are required.
>
> Also, I think both pebs_active and pebs_vmx can he a static_branch, but
> that can be done later I suppose.
Hi Peter,

If I understand this correctly, are you suggesting we put "if (!(guest & 
PERF_GUEST_ACTIVE))" first because this is a lower cost branch?

Thanks,
Zhu Lingshan