linux-kernel - [RFC PATCH v6 12/32] x86/apic: Unionize apic regs for 32bit/64bit access w/o type casting

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Message-ID: <20250514071803.209166-13-Neeraj.Upadhyay@amd.com>
Date: Wed, 14 May 2025 12:47:43 +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 12/32] x86/apic: Unionize apic regs for 32bit/64bit access w/o type casting

Define apic_page construct to unionize apic register 32bit/64bit
accesses and use it in apic_{get|set}*() to avoid using type
casting. Also, change 'regs' parameter to void pointer for automatic
pointer type conversion.

One caveat of this change is that the generated code is slighly
larger. Below is the code generation for apic_get_reg() using
gcc-14.2:

* Without change:

apic_get_reg:

89 f6       mov    esi,esi
8b 04 37    mov    eax,DWORD PTR [rdi+rsi*1]
c3          ret

* With change:

apic_get_reg:

c1 ee 02    shr    esi,0x2
8b 04 b7    mov    eax,DWORD PTR [rdi+rsi*4]
c3          ret

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
---
Changes since v5:

 - New change which is refactored out from v5 to apic.h.

 arch/x86/include/asm/apic.h | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index b377718d34d3..ea43e2f4c1c8 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -525,26 +525,43 @@ static inline int apic_find_highest_vector(void *bitmap)
 	return -1;
 }
 
-static inline u32 apic_get_reg(char *regs, unsigned int reg)
+struct apic_page {
+	union {
+		u64     regs64[PAGE_SIZE / 8];
+		u32     regs[PAGE_SIZE / 4];
+		u8      bytes[PAGE_SIZE];
+	};
+} __aligned(PAGE_SIZE);
+
+static inline u32 apic_get_reg(void *regs, unsigned int reg)
 {
-	return *((u32 *) (regs + reg));
+	struct apic_page *ap = regs;
+
+	return ap->regs[reg / 4];
 }
 
-static inline void apic_set_reg(char *regs, unsigned int reg, u32 val)
+static inline void apic_set_reg(void *regs, unsigned int reg, u32 val)
 {
-	*((u32 *) (regs + reg)) = val;
+	struct apic_page *ap = regs;
+
+	ap->regs[reg / 4] = val;
 }
 
-static __always_inline u64 apic_get_reg64(char *regs, unsigned int reg)
+static __always_inline u64 apic_get_reg64(void *regs, unsigned int reg)
 {
+	struct apic_page *ap = regs;
+
 	BUILD_BUG_ON(reg != APIC_ICR);
-	return *((u64 *) (regs + reg));
+
+	return ap->regs64[reg / 8];
 }
 
-static __always_inline void apic_set_reg64(char *regs, unsigned int reg, u64 val)
+static __always_inline void apic_set_reg64(void *regs, unsigned int reg, u64 val)
 {
+	struct apic_page *ap = regs;
+
 	BUILD_BUG_ON(reg != APIC_ICR);
-	*((u64 *) (regs + reg)) = val;
+	ap->regs64[reg / 8] = val;
 }
 
 static inline void apic_clear_vector(unsigned int vec, void *bitmap)
-- 
2.34.1