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Message-ID: <20231017162811.200569-3-ubizjak@gmail.com>
Date: Tue, 17 Oct 2023 18:27:34 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org, xen-devel@...ts.xenproject.org,
linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH -tip v3 3/3] x86/percpu: Introduce %rip-relative addressing to PER_CPU_VAR
Introduce x86_64 %rip-relative addressing to PER_CPU_VAR macro.
Instructions using %rip-relative address operand are one byte shorter
than their absolute address counterparts and are also compatible with
position independent executable (-fpie) build. The patch reduces
code size of a test kernel build by 150 bytes.
PER_CPU_VAR macro is intended to be applied to a symbol and should
not be used with register operands. Introduce new __percpu macro and
use it in cmpxchg{8,16}b_emu.S instead.
Also add a missing function comment to this_cpu_cmpxchg8b_emu.
No functional changes intended.
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
--
v2: Introduce PER_CPU_ARG macro to conditionally enable
segment registers in cmpxchg{8,16}b_emu.S for CONFIG_SMP.
v3: Introduce __percpu macro instead of PER_CPU_ARG (hpa).
---
arch/x86/include/asm/percpu.h | 12 ++++++++----
arch/x86/lib/cmpxchg16b_emu.S | 12 ++++++------
arch/x86/lib/cmpxchg8b_emu.S | 30 +++++++++++++++++++++---------
3 files changed, 35 insertions(+), 19 deletions(-)
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 54746903b8c3..02f1780f02f5 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -4,17 +4,21 @@
#ifdef CONFIG_X86_64
#define __percpu_seg gs
+#define __percpu_rel (%rip)
#else
#define __percpu_seg fs
+#define __percpu_rel
#endif
#ifdef __ASSEMBLY__
#ifdef CONFIG_SMP
-#define PER_CPU_VAR(var) %__percpu_seg:var
-#else /* ! SMP */
-#define PER_CPU_VAR(var) var
-#endif /* SMP */
+#define __percpu %__percpu_seg:
+#else
+#define __percpu
+#endif
+
+#define PER_CPU_VAR(var) __percpu(var)__percpu_rel
#ifdef CONFIG_X86_64_SMP
#define INIT_PER_CPU_VAR(var) init_per_cpu__##var
diff --git a/arch/x86/lib/cmpxchg16b_emu.S b/arch/x86/lib/cmpxchg16b_emu.S
index 6962df315793..4fb44894ad87 100644
--- a/arch/x86/lib/cmpxchg16b_emu.S
+++ b/arch/x86/lib/cmpxchg16b_emu.S
@@ -23,14 +23,14 @@ SYM_FUNC_START(this_cpu_cmpxchg16b_emu)
cli
/* if (*ptr == old) */
- cmpq PER_CPU_VAR(0(%rsi)), %rax
+ cmpq __percpu (%rsi), %rax
jne .Lnot_same
- cmpq PER_CPU_VAR(8(%rsi)), %rdx
+ cmpq __percpu 8(%rsi), %rdx
jne .Lnot_same
/* *ptr = new */
- movq %rbx, PER_CPU_VAR(0(%rsi))
- movq %rcx, PER_CPU_VAR(8(%rsi))
+ movq %rbx, __percpu (%rsi)
+ movq %rcx, __percpu 8(%rsi)
/* set ZF in EFLAGS to indicate success */
orl $X86_EFLAGS_ZF, (%rsp)
@@ -42,8 +42,8 @@ SYM_FUNC_START(this_cpu_cmpxchg16b_emu)
/* *ptr != old */
/* old = *ptr */
- movq PER_CPU_VAR(0(%rsi)), %rax
- movq PER_CPU_VAR(8(%rsi)), %rdx
+ movq __percpu (%rsi), %rax
+ movq __percpu 8(%rsi), %rdx
/* clear ZF in EFLAGS to indicate failure */
andl $(~X86_EFLAGS_ZF), (%rsp)
diff --git a/arch/x86/lib/cmpxchg8b_emu.S b/arch/x86/lib/cmpxchg8b_emu.S
index 873e4ef23e49..1c96be769adc 100644
--- a/arch/x86/lib/cmpxchg8b_emu.S
+++ b/arch/x86/lib/cmpxchg8b_emu.S
@@ -24,12 +24,12 @@ SYM_FUNC_START(cmpxchg8b_emu)
pushfl
cli
- cmpl 0(%esi), %eax
+ cmpl (%esi), %eax
jne .Lnot_same
cmpl 4(%esi), %edx
jne .Lnot_same
- movl %ebx, 0(%esi)
+ movl %ebx, (%esi)
movl %ecx, 4(%esi)
orl $X86_EFLAGS_ZF, (%esp)
@@ -38,7 +38,7 @@ SYM_FUNC_START(cmpxchg8b_emu)
RET
.Lnot_same:
- movl 0(%esi), %eax
+ movl (%esi), %eax
movl 4(%esi), %edx
andl $(~X86_EFLAGS_ZF), (%esp)
@@ -53,18 +53,30 @@ EXPORT_SYMBOL(cmpxchg8b_emu)
#ifndef CONFIG_UML
+/*
+ * Emulate 'cmpxchg8b %fs:(%rsi)'
+ *
+ * Inputs:
+ * %esi : memory location to compare
+ * %eax : low 32 bits of old value
+ * %edx : high 32 bits of old value
+ * %ebx : low 32 bits of new value
+ * %ecx : high 32 bits of new value
+ *
+ * Notably this is not LOCK prefixed and is not safe against NMIs
+ */
SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
pushfl
cli
- cmpl PER_CPU_VAR(0(%esi)), %eax
+ cmpl __percpu (%esi), %eax
jne .Lnot_same2
- cmpl PER_CPU_VAR(4(%esi)), %edx
+ cmpl __percpu 4(%esi), %edx
jne .Lnot_same2
- movl %ebx, PER_CPU_VAR(0(%esi))
- movl %ecx, PER_CPU_VAR(4(%esi))
+ movl %ebx, __percpu (%esi)
+ movl %ecx, __percpu 4(%esi)
orl $X86_EFLAGS_ZF, (%esp)
@@ -72,8 +84,8 @@ SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
RET
.Lnot_same2:
- movl PER_CPU_VAR(0(%esi)), %eax
- movl PER_CPU_VAR(4(%esi)), %edx
+ movl __percpu (%esi), %eax
+ movl __percpu 4(%esi), %edx
andl $(~X86_EFLAGS_ZF), (%esp)
--
2.41.0
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