linux-kernel - [PATCH RESEND] x86/uaccess: Improve __try_cmpxchg64_user_asm for x86

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Date:   Tue, 28 Jun 2022 18:16:12 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH RESEND] x86/uaccess: Improve __try_cmpxchg64_user_asm for x86_32

Improve __try_cmpxcgh64_user_asm for !CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT
by relaxing the output register constraint from "c" to "q" constraint,
which allows the compiler to choose between %ecx or %ebx register.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Reviewed-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/include/asm/uaccess.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 913e593a3b45..b0583c1da14f 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -448,7 +448,7 @@ do {									\
 
 #ifdef CONFIG_X86_32
 /*
- * Unlike the normal CMPXCHG, hardcode ECX for both success/fail and error.
+ * Unlike the normal CMPXCHG, use output GPR for both success/fail and error.
  * There are only six GPRs available and four (EAX, EBX, ECX, and EDX) are
  * hardcoded by CMPXCHG8B, leaving only ESI and EDI.  If the compiler uses
  * both ESI and EDI for the memory operand, compilation will fail if the error
@@ -461,11 +461,12 @@ do {									\
 	__typeof__(*(_ptr)) __new = (_new);				\
 	asm volatile("\n"						\
 		     "1: " LOCK_PREFIX "cmpxchg8b %[ptr]\n"		\
-		     "mov $0, %%ecx\n\t"				\
-		     "setz %%cl\n"					\
+		     "mov $0, %[result]\n\t"				\
+		     "setz %b[result]\n"				\
 		     "2:\n"						\
-		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG, %%ecx) \
-		     : [result]"=c" (__result),				\
+		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG,	\
+					   %[result])			\
+		     : [result] "=q" (__result),			\
 		       "+A" (__old),					\
 		       [ptr] "+m" (*_ptr)				\
 		     : "b" ((u32)__new),				\
-- 
2.35.3