From: Thomas Gleixner <tglx@linutronix.de>
To: Sean Christopherson <seanjc@google.com>
Cc: Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Jacob Pan <jacob.jun.pan@linux.intel.com>,
Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial loop (to load PIR vals)
Date: Tue, 18 Mar 2025 10:27:23 +0100 [thread overview]
Message-ID: <87ldt24rk4.ffs@tglx> (raw)
In-Reply-To: <Z9hThFNFrrbXjkjc@google.com>
On Mon, Mar 17 2025 at 09:53, Sean Christopherson wrote:
> On Mon, Mar 17, 2025, Thomas Gleixner wrote:
>> > - 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.
Neither does GCC 12. That's why I noticed.
next prev parent reply other threads:[~2025-03-18 9:27 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-03-15 3:06 [PATCH 0/8] x86/irq: KVM: Optimize KVM's PIR harvesting Sean Christopherson
2025-03-15 3:06 ` [PATCH 1/8] x86/irq: Ensure initial PIR loads are performed exactly once Sean Christopherson
2025-03-17 11:23 ` Thomas Gleixner
2025-03-15 3:06 ` [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial loop (to load PIR vals) Sean Christopherson
2025-03-17 13:13 ` Thomas Gleixner
2025-03-17 16:53 ` Sean Christopherson
2025-03-18 9:27 ` Thomas Gleixner [this message]
2025-03-15 3:06 ` [PATCH 3/8] KVM: VMX: Ensure vIRR isn't reloaded at odd times when sync'ing PIR Sean Christopherson
2025-03-15 3:06 ` [PATCH 4/8] x86/irq: KVM: Track PIR bitmap as an "unsigned long" array Sean Christopherson
2025-03-15 3:06 ` [PATCH 5/8] KVM: VMX: Process PIR using 64-bit accesses on 64-bit kernels Sean Christopherson
2025-03-15 3:06 ` [PATCH 6/8] KVM: VMX: Isolate pure loads from atomic XCHG when processing PIR Sean Christopherson
2025-03-15 3:06 ` [PATCH 7/8] KVM: VMX: Use arch_xchg() when processing PIR to avoid instrumentation Sean Christopherson
2025-03-15 3:06 ` [PATCH 8/8] x86/irq: KVM: Add helper for harvesting PIR to deduplicate KVM and posted MSIs Sean Christopherson
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