From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Mon, 7 Apr 2025 15:10:57 +0000 (+0100)
Subject: x86/AMD: Convert rdmsr_amd_safe() to use asm goto()
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=2fdeed237249f365b42cac7039e1ae6c7b1b60db;p=people%2Fandrewcoop%2Fxen.git

x86/AMD: Convert rdmsr_amd_safe() to use asm goto()

Unlike the WRMSR side, we can't use asm goto() unconditionally, because our
toolchain baseline doesn't support asm goto with outputs.

However, the code generation improvements are substantial enough to warrant
the duplicate implementations.

Detect support, and use asm goto() when available.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---

diff --git a/xen/Kconfig b/xen/Kconfig
index ae1c401a98..486b36b843 100644
--- a/xen/Kconfig
+++ b/xen/Kconfig
@@ -33,6 +33,20 @@ config LD_IS_LLVM
 config CC_SPLIT_SECTIONS
 	bool
 
+# Fixed in GCC 14, 13.3, 12.4 and 11.5
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921
+config GCC_ASM_GOTO_OUTPUT_BROKEN
+	bool
+	depends on CC_IS_GCC
+	default y if GCC_VERSION < 110500
+	default y if GCC_VERSION >= 120000 && GCC_VERSION < 120400
+	default y if GCC_VERSION >= 130000 && GCC_VERSION < 130300
+
+config CC_HAS_ASM_GOTO_OUTPUT
+	def_bool y
+	depends on !GCC_ASM_GOTO_OUTPUT_BROKEN
+	depends on $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)
+
 # Set code alignment.
 #
 # Allow setting on a boolean basis, and then convert such selection to an
diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index ce4e1df710..37d67dd15c 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -59,6 +59,18 @@ static bool __read_mostly fam17_c6_disabled;
 static inline int rdmsr_amd_safe(unsigned int msr, unsigned int *lo,
 				 unsigned int *hi)
 {
+#ifdef CONFIG_CC_HAS_ASM_GOTO_OUTPUT
+    asm goto ( "1: rdmsr\n\t"
+               _ASM_EXTABLE(1b, %l[fault])
+               : "=a" (*lo), "=d" (*hi)
+               : "c" (msr), "D" (0x9c5a203a)
+               :
+               : fault );
+    return 0;
+
+ fault:
+    return -EFAULT;
+#else
 	int err;
 
 	asm volatile("1: rdmsr\n2:\n"
@@ -71,6 +83,7 @@ static inline int rdmsr_amd_safe(unsigned int msr, unsigned int *lo,
 		     : "c" (msr), "D" (0x9c5a203a), "2" (0), "i" (-EFAULT));
 
 	return err;
+#endif
 }
 
 static inline int wrmsr_amd_safe(unsigned int msr, unsigned int lo,