linux-kernel - Re: [PATCH v4 11/38] perf/x86: Forbid PMI handler when guest own PMU

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Message-ID: <aCUukXIC_9cxHQd3@google.com>
Date: Wed, 14 May 2025 17:00:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Mingwei Zhang <mizhang@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com, 
	Kan <kan.liang@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Yongwei Ma <yongwei.ma@...el.com>, Xiong Zhang <xiong.y.zhang@...ux.intel.com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, Jim Mattson <jmattson@...gle.com>, 
	Sandipan Das <sandipan.das@....com>, Zide Chen <zide.chen@...el.com>, 
	Eranian Stephane <eranian@...gle.com>, Shukla Manali <Manali.Shukla@....com>, 
	Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 11/38] perf/x86: Forbid PMI handler when guest own PMU

On Mon, Mar 24, 2025, Mingwei Zhang wrote:
> If a guest PMI is delivered after VM-exit, the KVM maskable interrupt will
> be held pending until EFLAGS.IF is set. In the meantime, if the logical
> processor receives an NMI for any reason at all, perf_event_nmi_handler()
> will be invoked. If there is any active perf event anywhere on the system,
> x86_pmu_handle_irq() will be invoked, and it will clear
> IA32_PERF_GLOBAL_STATUS. By the time KVM's PMI handler is invoked, it will
> be a mystery which counter(s) overflowed.
> 
> When LVTPC is using KVM PMI vecotr, PMU is owned by guest, Host NMI let
> x86_pmu_handle_irq() run, x86_pmu_handle_irq() restore PMU vector to NMI
> and clear IA32_PERF_GLOBAL_STATUS, this breaks guest vPMU passthrough
> environment.
> 
> So modify perf_event_nmi_handler() to check perf_in_guest per cpu variable,
> and if so, to simply return without calling x86_pmu_handle_irq().
> 
> Suggested-by: Jim Mattson <jmattson@...gle.com>
> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> ---
>  arch/x86/events/core.c | 27 +++++++++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 28161d6ff26d..96a173bbbec2 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -54,6 +54,8 @@ DEFINE_PER_CPU(struct cpu_hw_events, cpu_hw_events) = {
>  	.pmu = &pmu,
>  };
>  
> +static DEFINE_PER_CPU(bool, pmi_vector_is_nmi) = true;

I strongly prefer guest_ctx_loaded.  pmi_vector_is_nmi very inflexible and
doesn't communicate *why* perf's NMI handler needs to ignore NMIs

>  DEFINE_STATIC_KEY_FALSE(rdpmc_never_available_key);
>  DEFINE_STATIC_KEY_FALSE(rdpmc_always_available_key);
>  DEFINE_STATIC_KEY_FALSE(perf_is_hybrid);
> @@ -1737,6 +1739,24 @@ perf_event_nmi_handler(unsigned int cmd, struct pt_regs *regs)
>  	u64 finish_clock;
>  	int ret;
>  
> +	/*
> +	 * When guest pmu context is loaded this handler should be forbidden from
> +	 * running, the reasons are:
> +	 * 1. After perf_guest_enter() is called, and before cpu enter into
> +	 *    non-root mode, host non-PMI NMI could happen, but x86_pmu_handle_irq()
> +	 *    restore PMU to use NMI vector, which destroy KVM PMI vector setting.
> +	 * 2. When VM is running, host non-PMI NMI causes VM exit, KVM will
> +	 *    call host NMI handler (vmx_vcpu_enter_exit()) first before KVM save
> +	 *    guest PMU context (kvm_pmu_put_guest_context()), as x86_pmu_handle_irq()
> +	 *    clear global_status MSR which has guest status now, then this destroy
> +	 *    guest PMU status.
> +	 * 3. After VM exit, but before KVM save guest PMU context, host non-PMI NMI
> +	 *    could happen, x86_pmu_handle_irq() clear global_status MSR which has
> +	 *    guest status now, then this destroy guest PMU status.
> +	 */

This *might* be useful for a changelog, but even then it's probably overkill.
NMIs can happen at any time, that's the full the story.  Enumerating the exact
edge cases adds a lot of noise and not much value.