[PATCH v2 1/4] x86/percpu: Introduce PER_CPU_ARG and use it in cmpxchg{8,16}b_emu.S

Uros Bizjak
2023-10-12T20:18:05Z

PER_CPU_VAR macro is intended to be applied to a symbol and should not
be used with general operands. Introduce new PER_CPU_ARG macro and
use it in cmpxchg{8,16}b_emu.S instead.

PER_CPU_VAR macro will be repurposed for %rip-relative addressing.

Also add a missing function comment to this_cpu_cmpxchg8b_emu.

No functional changes intended.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
--
v2: Introduce PER_CPU_ARG macro to conditionally enable
segment registers in cmpxchg{8,16}b_emu.S for CONFIG_SMP.
---
arch/x86/include/asm/percpu.h | 2 ++
arch/x86/lib/cmpxchg16b_emu.S | 12 ++++++------
arch/x86/lib/cmpxchg8b_emu.S | 24 ++++++++++++++++++------
3 files changed, 26 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 34734d730463..83e6a4bcea38 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -11,8 +11,10 @@
#ifdef __ASSEMBLY__

#ifdef CONFIG_SMP
+#define PER_CPU_ARG(arg) %__percpu_seg:arg
#define PER_CPU_VAR(var) %__percpu_seg:var
#else /* ! SMP */
+#define PER_CPU_ARG(arg) arg
#define PER_CPU_VAR(var) var
#endif /* SMP */

diff --git a/arch/x86/lib/cmpxchg16b_emu.S b/arch/x86/lib/cmpxchg16b_emu.S
index 6962df315793..b6b942d07a00 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 PER_CPU_ARG(0(%rsi)), %rax
jne .Lnot_same
- cmpq PER_CPU_VAR(8(%rsi)), %rdx
+ cmpq PER_CPU_ARG(8(%rsi)), %rdx
jne .Lnot_same

/* *ptr = new */
- movq %rbx, PER_CPU_VAR(0(%rsi))
- movq %rcx, PER_CPU_VAR(8(%rsi))
+ movq %rbx, PER_CPU_ARG(0(%rsi))
+ movq %rcx, PER_CPU_ARG(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 PER_CPU_ARG(0(%rsi)), %rax
+ movq PER_CPU_ARG(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 49805257b125..9a0a7feeaf7c 100644
--- a/arch/x86/lib/cmpxchg8b_emu.S
+++ b/arch/x86/lib/cmpxchg8b_emu.S
@@ -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 PER_CPU_ARG(0(%esi)), %eax
jne .Lnot_same2
- cmpl PER_CPU_VAR(4(%esi)), %edx
+ cmpl PER_CPU_ARG(4(%esi)), %edx
jne .Lnot_same2

- movl %ebx, PER_CPU_VAR(0(%esi))
- movl %ecx, PER_CPU_VAR(4(%esi))
+ movl %ebx, PER_CPU_ARG(0(%esi))
+ movl %ecx, PER_CPU_ARG(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 PER_CPU_ARG(0(%esi)), %eax
+ movl PER_CPU_ARG(4(%esi)), %edx

andl $(~X86_EFLAGS_ZF), (%esp)

--
2.41.0