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: <Z9hThFNFrrbXjkjc@google.com>
Date: Mon, 17 Mar 2025 09:53:24 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, 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 Mon, Mar 17, 2025, Thomas Gleixner wrote:
> 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.

Huh.  gcc elides the conditional branches when computing found_irq regardless of
the approach; the JEs in the changelog are from skipping the XCHG.

But clang-14 does not.  I'll slot this in.

Thanks!