linux-kernel - Re: [PATCH v4 06/38] x86/irq: Factor out common code for installing kvm irq handler

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Message-ID: <aCUlbDNoxQ-65mc0@google.com>
Date: Wed, 14 May 2025 16:21:16 -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 06/38] x86/irq: Factor out common code for installing
 kvm irq handler

On Mon, Mar 24, 2025, Mingwei Zhang wrote:
> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> index 385e3a5fc304..18cd418fe106 100644
> --- a/arch/x86/kernel/irq.c
> +++ b/arch/x86/kernel/irq.c
> @@ -312,16 +312,22 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_x86_platform_ipi)
>  static void dummy_handler(void) {}
>  static void (*kvm_posted_intr_wakeup_handler)(void) = dummy_handler;
>  
> -void kvm_set_posted_intr_wakeup_handler(void (*handler)(void))
> +void x86_set_kvm_irq_handler(u8 vector, void (*handler)(void))
>  {
> -	if (handler)
> +	if (!handler)
> +		handler = dummy_handler;
> +
> +	if (vector == POSTED_INTR_WAKEUP_VECTOR &&
> +	    (handler == dummy_handler ||
> +	     kvm_posted_intr_wakeup_handler == dummy_handler))
>  		kvm_posted_intr_wakeup_handler = handler;
> -	else {
> -		kvm_posted_intr_wakeup_handler = dummy_handler;
> +	else
> +		WARN_ON_ONCE(1);
> +
> +	if (handler == dummy_handler)

Eww.  Aside from the fact that the dummy_handler implementation is pointless
overhead, I don't think KVM should own the IRQ vector.  Given that perf owns the
LVTPC, i.e. responsible for switching between NMI and the medited PMI IRQ, I
think perf should also own the vector.  KVM can then use the existing perf guest
callbacks to wire up its PMI handler.

And with that, this patch can be dropped.