x86/cpu: Add Erratum 88 detection on AMD

Erratum 88 affects old AMD K8s, where a SWAPGS fails to cause an input
dependency on GS. Therefore, we need to MFENCE before it.

But that MFENCE is expensive and unnecessary on the remaining x86 CPUs
out there so patch it out on the CPUs which don't require it.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rudolf Marek <r.marek@assembler.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/aec6b2df1bfc56101d4e9e2e5d5d570bf41663c6.1460075211.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64.S

@@ -783,7 +783,7 @@ ENTRY(native_load_gs_index)
 	SWAPGS
 gs_change:
 	movl	%edi, %gs
-2:	mfence					/* workaround */
+2:	ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
 	SWAPGS
 	popfq
 	ret

arch/x86/include/asm/cpufeatures.h

@@ -295,6 +295,8 @@
 #define X86_BUG_CLFLUSH_MONITOR	X86_BUG(7) /* AAI65, CLFLUSH required before MONITOR */
 #define X86_BUG_SYSRET_SS_ATTRS	X86_BUG(8) /* SYSRET doesn't fix up SS attrs */
 #define X86_BUG_NULL_SEG	X86_BUG(9) /* Nulling a selector preserves the base */
+#define X86_BUG_SWAPGS_FENCE	X86_BUG(10) /* SWAPGS without input dep on GS */
+
 
 #ifdef CONFIG_X86_32
 /*

arch/x86/kernel/cpu/amd.c

@@ -632,6 +632,7 @@ static void init_amd_k8(struct cpuinfo_x86 *c)
 	 */
 	msr_set_bit(MSR_K7_HWCR, 6);
 #endif
+	set_cpu_bug(c, X86_BUG_SWAPGS_FENCE);
 }
 
 static void init_amd_gh(struct cpuinfo_x86 *c)