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

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Message-ID: <434055d7-e7ec-48ad-9ef5-2d80bb824a63@linux.intel.com>
Date: Thu, 15 May 2025 10:10:40 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 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>,
 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 5/15/2025 7:21 AM, Sean Christopherson wrote:
> 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.

Hmm, yes, make sense.

>
> And with that, this patch can be dropped.
>