This workaround is taken from Linux, and the main motivation (besides
such workarounds indeed belonging in the hypervisor rather than each
kernel) is to suppress the warnings in the Xen log each Linux guest
would cause due to the disallowed wrmsr.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -43,6 +43,47 @@ static inline void wrmsr_amd(unsigned in
 	);
 }
 
+static inline int rdmsr_amd_safe(unsigned int msr, unsigned int *lo,
+				 unsigned int *hi)
+{
+	int err;
+
+	asm volatile("1: rdmsr\n2:\n"
+		     ".section .fixup,\"ax\"\n"
+		     "3: movl %6,%2\n"
+		     "   jmp 2b\n"
+		     ".previous\n"
+		     ".section __ex_table,\"a\"\n"
+		     __FIXUP_ALIGN "\n"
+		     __FIXUP_WORD " 1b,3b\n"
+		     ".previous\n"
+		     : "=a" (*lo), "=d" (*hi), "=r" (err)
+		     : "c" (msr), "D" (0x9c5a203a), "2" (0), "i" (-EFAULT));
+
+	return err;
+}
+
+static inline int wrmsr_amd_safe(unsigned int msr, unsigned int lo,
+				 unsigned int hi)
+{
+	int err;
+
+	asm volatile("1: wrmsr\n2:\n"
+		     ".section .fixup,\"ax\"\n"
+		     "3: movl %6,%0\n"
+		     "   jmp 2b\n"
+		     ".previous\n"
+		     ".section __ex_table,\"a\"\n"
+		     __FIXUP_ALIGN "\n"
+		     __FIXUP_WORD " 1b,3b\n"
+		     ".previous\n"
+		     : "=r" (err)
+		     : "c" (msr), "a" (lo), "d" (hi), "D" (0x9c5a203a),
+		       "0" (0), "i" (-EFAULT));
+
+	return err;
+}
+
 /*
  * Mask the features and extended features returned by CPUID.  Parameters are
  * set from the boot line via two methods:
@@ -329,6 +370,24 @@ static void __devinit init_amd(struct cp
 	   3DNow is IDd by bit 31 in extended CPUID (1*32+31) anyway */
 	clear_bit(0*32+31, c->x86_capability);
 	
+#ifdef CONFIG_X86_64
+	if (c->x86 == 0xf && c->x86_model < 0x14
+	    && cpu_has(c, X86_FEATURE_LAHF_LM)) {
+		/*
+		 * Some BIOSes incorrectly force this feature, but only K8
+		 * revision D (model = 0x14) and later actually support it.
+		 * (AMD Erratum #110, docId: 25759).
+		 */
+		unsigned int lo, hi;
+
+		clear_bit(X86_FEATURE_LAHF_LM, c->x86_capability);
+		if (!rdmsr_amd_safe(0xc001100d, &lo, &hi)) {
+			hi &= ~1;
+			wrmsr_amd_safe(0xc001100d, lo, hi);
+		}
+	}
+#endif
+
 	r = get_model_name(c);
 
 	switch(c->x86)