linux-kernel - [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: <20250315030630.2371712-3-seanjc@google.com>
Date: Fri, 14 Mar 2025 20:06:23 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>, 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: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial
 loop (to load PIR vals)

Track whether or not at least one IRQ was found in PIR during the initial
loop to load PIR chunks from memory.  Doing so generates slightly better
code (arguably), especially for the case where there are no pending IRQs.

Note, while PIR can be modified between the initial load and the XCHG, it
can only _gain_ new IRQs, i.e. there is no danger of a false positive due
to the final version of pir_copy[] being empty.

Opportunistically rename the boolean in anticipation of moving the PIR
accesses to a common helper that can be shared by posted MSIs and KVM.

Old:
   <+74>:	test   %rdx,%rdx
   <+77>:	je     0xffffffff812bbeb0 <handle_pending_pir+144>
   <pir[0]>
   <+88>:	mov    $0x1,%dl>
   <+90>:	test   %rsi,%rsi
   <+93>:	je     0xffffffff812bbe8c <handle_pending_pir+108>
   <pir[1]>
   <+106>:	mov    $0x1,%dl
   <+108>:	test   %rcx,%rcx
   <+111>:	je     0xffffffff812bbe9e <handle_pending_pir+126>
   <pir[2]>
   <+124>:	mov    $0x1,%dl
   <+126>:	test   %rax,%rax
   <+129>:	je     0xffffffff812bbeb9 <handle_pending_pir+153>
   <pir[3]>
   <+142>:	jmp    0xffffffff812bbec1 <handle_pending_pir+161>
   <+144>:	xor    %edx,%edx
   <+146>:	test   %rsi,%rsi
   <+149>:	jne    0xffffffff812bbe7f <handle_pending_pir+95>
   <+151>:	jmp    0xffffffff812bbe8c <handle_pending_pir+108>
   <+153>:	test   %dl,%dl
   <+155>:	je     0xffffffff812bbf8e <handle_pending_pir+366>

New:
   <+74>:	mov    %rax,%r8
   <+77>:	or     %rcx,%r8
   <+80>:	or     %rdx,%r8
   <+83>:	or     %rsi,%r8
   <+86>:	setne  %bl
   <+89>:	je     0xffffffff812bbf88 <handle_pending_pir+360>
   <+95>:	test   %rsi,%rsi
   <+98>:	je     0xffffffff812bbe8d <handle_pending_pir+109>
   <pir[0]>
   <+109>:	test   %rdx,%rdx
   <+112>:	je     0xffffffff812bbe9d <handle_pending_pir+125>
   <pir[1]>
   <+125>:	test   %rcx,%rcx
   <+128>:	je     0xffffffff812bbead <handle_pending_pir+141>
   <pir[2]>
   <+141>:	test   %rax,%rax
   <+144>:	je     0xffffffff812bbebd <handle_pending_pir+157>
   <pir[3]>

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kernel/irq.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 9e5263887ff6..3f95b00ccd7f 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -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;
+	}
+
+	if (!found_irq)
+		return false;
 
 	for (i = 0; i < 4; i++) {
 		if (!pir_copy[i])
 			continue;
 
 		pir_copy[i] = arch_xchg(&pir[i], 0);
-		handled = true;
 	}
 
-	if (handled) {
-		for_each_set_bit_from(vec, pir_copy, FIRST_SYSTEM_VECTOR)
-			call_irq_handler(vec, regs);
-	}
+	for_each_set_bit_from(vec, pir_copy, FIRST_SYSTEM_VECTOR)
+		call_irq_handler(vec, regs);
 
-	return handled;
+	return true;
 }
 
 /*
-- 
2.49.0.rc1.451.g8f38331e32-goog