x86/srso: Add IBPB_BRTYPE support

Add support for the synthetic CPUID flag which "if this bit is 1,
it indicates that MSR 49h (PRED_CMD) bit 0 (IBPB) flushes all branch
type predictions from the CPU branch predictor."

This flag is there so that this capability in guests can be detected
easily (otherwise one would have to track microcode revisions which is
impossible for guests).

It is also needed only for Zen3 and -4. The other two (Zen1 and -2)
always flush branch type predictions by default.
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>

arch/x86/include/asm/cpufeatures.h

@@ -445,6 +445,8 @@
 #define X86_FEATURE_AUTOIBRS		(20*32+ 8) /* "" Automatic IBRS */
 #define X86_FEATURE_NO_SMM_CTL_MSR	(20*32+ 9) /* "" SMM_CTL MSR is not present */
 
+#define X86_FEATURE_IBPB_BRTYPE		(20*32+28) /* "" MSR_PRED_CMD[IBPB] flushes all branch type predictions */
+
 /*
  * BUG word(s)
  */

arch/x86/kernel/cpu/bugs.c

@@ -2238,10 +2238,20 @@ static void __init srso_select_mitigation(void)
 	if (!boot_cpu_has_bug(X86_BUG_SRSO) || cpu_mitigations_off())
 		return;
 
-	has_microcode = cpu_has_ibpb_brtype_microcode();
+	/*
+	 * The first check is for the kernel running as a guest in order
+	 * for guests to verify whether IBPB is a viable mitigation.
+	 */
+	has_microcode = boot_cpu_has(X86_FEATURE_IBPB_BRTYPE) || cpu_has_ibpb_brtype_microcode();
 	if (!has_microcode) {
 		pr_warn("IBPB-extending microcode not applied!\n");
 		pr_warn(SRSO_NOTICE);
+	} else {
+		/*
+		 * Enable the synthetic (even if in a real CPUID leaf)
+		 * flag for guests.
+		 */
+		setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE);
 	}
 
 	switch (srso_cmd) {