[PATCH] x86/asm: Switch clflush alternatives to use %a address operand modifier

Uros Bizjak
2026-03-18T09:08:42Z

The inline asm used with alternative_input() specifies the address
operand for clflush with the "a" input operand constraint and
explicit "(%[addr])" dereference:

"clflush (%[addr])", [addr] "a" (addr)

This forces the pointer into %rax and manually encodes the memory
operand in the template. Instead, use the %a address operand
modifier and relax the constraint from "a" to "r":

"clflush %a[addr]", [addr] "r" (addr)

This lets the compiler choose the register while generating the
correct addressing mode.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
arch/x86/include/asm/mwait.h | 3 ++-
arch/x86/kernel/process.c | 3 ++-
2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index e4815e15dc9a..fcb7299b293a 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -116,7 +116,8 @@ static __always_inline void mwait_idle_with_hints(u32 eax, u32 ecx)
if (static_cpu_has_bug(X86_BUG_MONITOR) || !current_set_polling_and_test()) {
const void *addr = &current_thread_info()->flags;

- alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr));
+ alternative_input("", "clflush %a[addr]",
+ X86_BUG_CLFLUSH_MONITOR, [addr] "r" (addr));
__monitor(addr, 0, 0);

if (need_resched())
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 4c718f8adc59..8e295fb19b10 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -921,7 +921,8 @@ static __cpuidle void mwait_idle(void)
if (!current_set_polling_and_test()) {
const void *addr = &current_thread_info()->flags;

- alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr));
+ alternative_input("", "clflush %a[addr]",
+ X86_BUG_CLFLUSH_MONITOR, [addr] "r" (addr));
__monitor(addr, 0, 0);
if (need_resched())
goto out;
--
2.53.0