x86/mce/inject: Only write MCA_MISC when a value has been supplied

The MCA_MISC register is used to control the MCA thresholding feature on
AMD systems. Therefore, it is not generally part of the error state that
a user would adjust when testing non-thresholding cases.

However, MCA_MISC is unconditionally written even if a user does not
supply a value. The default value of '0' will be used and clobber the
register.

Write the MCA_MISC register only if the user has given a value for it.
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240523155641.2805411-2-yazen.ghannam@amd.com

arch/x86/kernel/cpu/mce/inject.c

@@ -487,12 +487,16 @@ static void prepare_msrs(void *info)
 			wrmsrl(MSR_AMD64_SMCA_MCx_ADDR(b), m.addr);
 		}
 
-		wrmsrl(MSR_AMD64_SMCA_MCx_MISC(b), m.misc);
 		wrmsrl(MSR_AMD64_SMCA_MCx_SYND(b), m.synd);
+
+		if (m.misc)
+			wrmsrl(MSR_AMD64_SMCA_MCx_MISC(b), m.misc);
 	} else {
 		wrmsrl(MSR_IA32_MCx_STATUS(b), m.status);
 		wrmsrl(MSR_IA32_MCx_ADDR(b), m.addr);
-		wrmsrl(MSR_IA32_MCx_MISC(b), m.misc);
+
+		if (m.misc)
+			wrmsrl(MSR_IA32_MCx_MISC(b), m.misc);
 	}
 }