linux-kernel - [RFC PATCH v6 01/32] x86/apic: KVM: Deduplicate APIC vector => register+bit math

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Message-ID: <20250514071803.209166-2-Neeraj.Upadhyay@amd.com>
Date: Wed, 14 May 2025 12:47:32 +0530
From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
To: <linux-kernel@...r.kernel.org>
CC: <bp@...en8.de>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<dave.hansen@...ux.intel.com>, <Thomas.Lendacky@....com>, <nikunj@....com>,
	<Santosh.Shukla@....com>, <Vasant.Hegde@....com>,
	<Suravee.Suthikulpanit@....com>, <David.Kaplan@....com>, <x86@...nel.org>,
	<hpa@...or.com>, <peterz@...radead.org>, <seanjc@...gle.com>,
	<pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<kirill.shutemov@...ux.intel.com>, <huibo.wang@....com>,
	<naveen.rao@....com>, <francescolavra.fl@...il.com>, <tiala@...rosoft.com>
Subject: [RFC PATCH v6 01/32] x86/apic: KVM: Deduplicate APIC vector => register+bit math

From: Sean Christopherson <seanjc@...gle.com>

Consolidate KVM's {REG,VEC}_POS() macros and lapic_vector_set_in_irr()'s
open coded equivalent logic in anticipation of the kernel gaining more
usage of vector => reg+bit lookups.

Use lapic_vector_set_in_irr()'s math as using divides for both the bit
number and register offset makes it easier to connect the dots, and for at
least one user, fixup_irqs(), "/ 32 * 0x10" generates ever so slightly
better code with gcc-14 (shaves a whole 3 bytes from the code stream):

((v) >> 5) << 4:
  c1 ef 05           shr    $0x5,%edi
  c1 e7 04           shl    $0x4,%edi
  81 c7 00 02 00 00  add    $0x200,%edi

(v) / 32 * 0x10:
  c1 ef 05           shr    $0x5,%edi
  83 c7 20           add    $0x20,%edi
  c1 e7 04           shl    $0x4,%edi

Keep KVM's tersely named macros as "wrappers" to avoid unnecessary churn
in KVM, and because the shorter names yield more readable code overall in
KVM.

No functional change intended (clang-19 and gcc-14 generate bit-for-bit
identical code for all of kvm.ko).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
---
Changes since v5:
 - New change.

 arch/x86/include/asm/apic.h | 7 +++++--
 arch/x86/kvm/lapic.h        | 4 ++--
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 68e10e30fe9b..7e0d3045e74c 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -488,11 +488,14 @@ static inline void apic_setup_apic_calls(void) { }
 
 extern void apic_ack_irq(struct irq_data *data);
 
+#define APIC_VECTOR_TO_BIT_NUMBER(v) ((v) % 32)
+#define APIC_VECTOR_TO_REG_OFFSET(v) ((v) / 32 * 0x10)
+
 static inline bool lapic_vector_set_in_irr(unsigned int vector)
 {
-	u32 irr = apic_read(APIC_IRR + (vector / 32 * 0x10));
+	u32 irr = apic_read(APIC_IRR + APIC_VECTOR_TO_REG_OFFSET(vector));
 
-	return !!(irr & (1U << (vector % 32)));
+	return !!(irr & (1U << APIC_VECTOR_TO_BIT_NUMBER(vector)));
 }
 
 static inline bool is_vector_pending(unsigned int vector)
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 1a8553ebdb42..05fdf88ef55a 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -143,8 +143,8 @@ void kvm_lapic_exit(void);
 
 u64 kvm_lapic_readable_reg_mask(struct kvm_lapic *apic);
 
-#define VEC_POS(v) ((v) & (32 - 1))
-#define REG_POS(v) (((v) >> 5) << 4)
+#define VEC_POS(v) APIC_VECTOR_TO_BIT_NUMBER(v)
+#define REG_POS(v) APIC_VECTOR_TO_REG_OFFSET(v)
 
 static inline void kvm_lapic_clear_vector(int vec, void *bitmap)
 {
-- 
2.34.1