x86/AMD: work around erratum 793

The recommendation is to set a bit in an MSR - do this if the firmware
didn't, considering that otherwise we expose ourselves to a guest
induced DoS.

This is CVE-2013-6885 / XSA-82.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>

--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -476,6 +476,20 @@ static void __devinit init_amd(struct cp
 		       "*** Pass \"allow_unsafe\" if you're trusting"
 		       " all your (PV) guest kernels. ***\n");
 
+	if (c->x86 == 0x16 && c->x86_model <= 0xf) {
+		rdmsrl(MSR_AMD64_LS_CFG, value);
+		if (!(value & (1 << 15))) {
+			static bool_t warned;
+
+			if (c == &boot_cpu_data || opt_cpu_info ||
+			    !test_and_set_bool(warned))
+				printk(KERN_WARNING
+				       "CPU%u: Applying workaround for erratum 793\n",
+				       smp_processor_id());
+			wrmsrl(MSR_AMD64_LS_CFG, value | (1 << 15));
+		}
+	}
+
 	/* AMD CPUs do not support SYSENTER outside of legacy mode. */
 	clear_bit(X86_FEATURE_SEP, c->x86_capability);
 
--- a/xen/include/asm-x86/msr-index.h
+++ b/xen/include/asm-x86/msr-index.h
@@ -213,6 +213,7 @@
 
 /* AMD64 MSRs */
 #define MSR_AMD64_NB_CFG		0xc001001f
+#define MSR_AMD64_LS_CFG		0xc0011020
 #define MSR_AMD64_IC_CFG		0xc0011021
 #define MSR_AMD64_DC_CFG		0xc0011022
 #define AMD64_NB_CFG_CF8_EXT_ENABLE_BIT	46