linux-kernel - Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial loop (to load PIR vals)

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Message-ID: <87wmcn4x78.ffs@tglx>
Date: Mon, 17 Mar 2025 14:13:15 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Sean Christopherson <seanjc@...gle.com>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, Jacob Pan
 <jacob.jun.pan@...ux.intel.com>, Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during
 initial loop (to load PIR vals)

On Fri, Mar 14 2025 at 20:06, Sean Christopherson wrote:
> @@ -409,25 +409,28 @@ static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
>  {
>  	int i, vec = FIRST_EXTERNAL_VECTOR;
>  	unsigned long pir_copy[4];
> -	bool handled = false;
> +	bool found_irq = false;
>  
> -	for (i = 0; i < 4; i++)
> +	for (i = 0; i < 4; i++) {
>  		pir_copy[i] = READ_ONCE(pir[i]);
> +		if (pir_copy[i])
> +			found_irq = true;
> +	}

That's four extra conditional branches. You can avoid them completely. See
delta patch below.

Thanks,

        tglx
---        
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -407,17 +407,15 @@ void intel_posted_msi_init(void)
  */
 static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
 {
+	unsigned long pir_copy[4], pend = 0;
 	int i, vec = FIRST_EXTERNAL_VECTOR;
-	unsigned long pir_copy[4];
-	bool found_irq = false;
 
 	for (i = 0; i < 4; i++) {
 		pir_copy[i] = READ_ONCE(pir[i]);
-		if (pir_copy[i])
-			found_irq = true;
+		pend |= pir_copy[i];
 	}
 
-	if (!found_irq)
+	if (!pend)
 		return false;
 
 	for (i = 0; i < 4; i++) {