linux-kernel - Re: [PATCH] perf/x86/amd: cpu_hw_events::perf_ctr_virt

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Date:   Mon, 11 Apr 2022 22:29:18 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Dongli Si <kvmx86@...il.com>
Cc:     liam.merwick@...cle.com, kim.phillips@....com, mingo@...nel.org,
        acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org, tglx@...utronix.de, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        jmattson@...gle.com, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Peter Zijlstra (Intel OTC, Netherlander)" <peterz@...radead.org>,
        "Joerg Roedel (KVM HoF)" <joro@...tes.org>
Subject: Re: [PATCH] perf/x86/amd: cpu_hw_events::perf_ctr_virt_mask should
 only be used on host

Dongli,

On 11/4/2022 9:46 pm, Dongli Si wrote:
> From: Dongli Si <sidongli1997@...il.com>
> 
> perf_ctr_virt_mask is used to mask Host-Only bit when SVM is disabled,
> Using it on a guest doesn't make sense and make things obscure.

Or you can work it out to make nested vPMU functional on AMD.

> 
> Revert commit df51fe7ea1c1c
> ("perf/x86/amd: Don't touch the AMD64_EVENTSEL_HOSTONLY bit inside the guest"),

This is not a typical revert commit.

> Because it make things a little obscure and this #GP has been fixed in KVM.

Please check the chronological order of the related commits and the motivations.

> 
> Fixes: 1018faa6cf23 ("perf/x86/kvm: Fix Host-Only/Guest-Only counting with SVM disabled")
> Signed-off-by: Dongli Si <sidongli1997@...il.com>
> ---
>   arch/x86/events/amd/core.c   | 19 ++++++++++++++++++-
>   arch/x86/events/perf_event.h |  3 +--
>   2 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> index 9687a8aef01c..5ac7d9410d36 100644
> --- a/arch/x86/events/amd/core.c
> +++ b/arch/x86/events/amd/core.c
> @@ -533,7 +533,12 @@ static void amd_pmu_cpu_starting(int cpu)
>   	struct amd_nb *nb;
>   	int i, nb_id;
>   
> -	cpuc->perf_ctr_virt_mask = AMD64_EVENTSEL_HOSTONLY;
> +	/*
> +	 * When SVM is disabled, set the Host-Only bit will cause the
> +	 * performance counter to not work.

It's ridiculous. Based on the AMD APM Table 13-3. Host/Guest Only Bits,
the performance counter would count "Host events" rather than "not work".

Note, your proposal change should work on the L0, L1 and L2.

> +	 */
> +	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +		cpuc->perf_ctr_virt_mask = AMD64_EVENTSEL_HOSTONLY;
>   
>   	if (!x86_pmu.amd_nb_constraints)
>   		return;
> @@ -1023,10 +1028,16 @@ __init int amd_pmu_init(void)
>   	return 0;
>   }
>   
> +/*
> + * Unmask the Host-only bit when SVM is enabled on the Host Hypervisor
> + */
>   void amd_pmu_enable_virt(void)
>   {
>   	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>   
> +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +		return;
> +
>   	cpuc->perf_ctr_virt_mask = 0;
>   
>   	/* Reload all events */
> @@ -1035,10 +1046,16 @@ void amd_pmu_enable_virt(void)
>   }
>   EXPORT_SYMBOL_GPL(amd_pmu_enable_virt);
>   
> +/*
> + * Mask the Host-only bit when SVM is disabled on the Host Hypervisor
> + */
>   void amd_pmu_disable_virt(void)
>   {
>   	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>   
> +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +		return;
> +
>   	/*
>   	 * We only mask out the Host-only bit so that host-only counting works
>   	 * when SVM is disabled. If someone sets up a guest-only counter when
> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
> index 150261d929b9..fa1428ca60b6 100644
> --- a/arch/x86/events/perf_event.h
> +++ b/arch/x86/events/perf_event.h
> @@ -1138,10 +1138,9 @@ void x86_pmu_stop(struct perf_event *event, int flags);
>   
>   static inline void x86_pmu_disable_event(struct perf_event *event)
>   {
> -	u64 disable_mask = __this_cpu_read(cpu_hw_events.perf_ctr_virt_mask);
>   	struct hw_perf_event *hwc = &event->hw;
>   
> -	wrmsrl(hwc->config_base, hwc->config & ~disable_mask);
> +	wrmsrl(hwc->config_base, hwc->config);
>   
>   	if (is_counter_pair(hwc))
>   		wrmsrl(x86_pmu_config_addr(hwc->idx + 1), 0);