x86/cpu/amd: Enumerate BTC_NO

BTC_NO indicates that hardware is not susceptible to Branch Type Confusion.

Zen3 CPUs don't suffer BTC.

Hypervisors are expected to synthesise BTC_NO when it is appropriate
given the migration pool, to prevent kernels using heuristics.

[ bp: Massage. ]

Signed-off-by: Andrew Cooper <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>

arch/x86/include/asm/cpufeatures.h

···
 #define X86_FEATURE_VIRT_SSBD		(13*32+25) /* Virtualized Speculative Store Bypass Disable */
 #define X86_FEATURE_AMD_SSB_NO		(13*32+26) /* "" Speculative Store Bypass is fixed in hardware. */
 #define X86_FEATURE_CPPC		(13*32+27) /* Collaborative Processor Performance Control */
+#define X86_FEATURE_BTC_NO		(13*32+29) /* "" Not vulnerable to Branch Type Confusion */
 #define X86_FEATURE_BRS			(13*32+31) /* Branch Sampling available */
 
 /* Thermal and Power Management Leaf, CPUID level 0x00000006 (EAX), word 14 */

arch/x86/kernel/cpu/amd.c

···
 	node_reclaim_distance = 32;
 #endif
 
-	/*
-	 * Fix erratum 1076: CPB feature bit not being set in CPUID.
-	 * Always set it, except when running under a hypervisor.
-	 */
-	if (!cpu_has(c, X86_FEATURE_HYPERVISOR) && !cpu_has(c, X86_FEATURE_CPB))
-		set_cpu_cap(c, X86_FEATURE_CPB);
+	/* Fix up CPUID bits, but only if not virtualised. */
+	if (!cpu_has(c, X86_FEATURE_HYPERVISOR)) {
+
+		/* Erratum 1076: CPB feature bit not being set in CPUID. */
+		if (!cpu_has(c, X86_FEATURE_CPB))
+			set_cpu_cap(c, X86_FEATURE_CPB);
+
+		/*
+		 * Zen3 (Fam19 model < 0x10) parts are not susceptible to
+		 * Branch Type Confusion, but predate the allocation of the
+		 * BTC_NO bit.
+		 */
+		if (c->x86 == 0x19 && !cpu_has(c, X86_FEATURE_BTC_NO))
+			set_cpu_cap(c, X86_FEATURE_BTC_NO);
+	}
 }
 
 static void init_amd(struct cpuinfo_x86 *c)

arch/x86/kernel/cpu/common.c

···
 	    !arch_cap_mmio_immune(ia32_cap))
 		setup_force_cpu_bug(X86_BUG_MMIO_STALE_DATA);
 
-	if ((cpu_matches(cpu_vuln_blacklist, RETBLEED) || (ia32_cap & ARCH_CAP_RSBA)))
-		setup_force_cpu_bug(X86_BUG_RETBLEED);
+	if (!cpu_has(c, X86_FEATURE_BTC_NO)) {
+		if (cpu_matches(cpu_vuln_blacklist, RETBLEED) || (ia32_cap & ARCH_CAP_RSBA))
+			setup_force_cpu_bug(X86_BUG_RETBLEED);
+	}
 
 	if (cpu_matches(cpu_vuln_whitelist, NO_MELTDOWN))
 		return;