x86/CPU/AMD: Improve the erratum 1386 workaround

Disable XSAVES only on machines which haven't loaded the microcode
revision containing the erratum fix.

This will come in handy when running archaic OSes as guests. OSes whose
brilliant programmers thought that CPUID is overrated and one should not
query it but use features directly, ala shoot first, ask questions
later... but only if you're alive after the shooting.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20240324200525.GBZgCHhYFsBj12PrKv@fat_crate.local

arch/x86/include/asm/cpu_device_id.h

@@ -190,6 +190,14 @@ struct x86_cpu_desc {
 	.x86_microcode_rev	= (revision),			\
 }
 
+#define AMD_CPU_DESC(fam, model, stepping, revision) {		\
+	.x86_family		= (fam),			\
+	.x86_vendor		= X86_VENDOR_AMD,		\
+	.x86_model		= (model),			\
+	.x86_stepping		= (stepping),			\
+	.x86_microcode_rev	= (revision),			\
+}
+
 extern const struct x86_cpu_id *x86_match_cpu(const struct x86_cpu_id *match);
 extern bool x86_cpu_has_min_microcode_rev(const struct x86_cpu_desc *table);
 

arch/x86/kernel/cpu/amd.c

@@ -13,6 +13,7 @@
 #include <asm/apic.h>
 #include <asm/cacheinfo.h>
 #include <asm/cpu.h>
+#include <asm/cpu_device_id.h>
 #include <asm/spec-ctrl.h>
 #include <asm/smp.h>
 #include <asm/numa.h>
@@ -802,6 +803,11 @@ static void init_amd_bd(struct cpuinfo_x86 *c)
 	clear_rdrand_cpuid_bit(c);
 }
 
+static const struct x86_cpu_desc erratum_1386_microcode[] = {
+	AMD_CPU_DESC(0x17,  0x1, 0x2, 0x0800126e),
+	AMD_CPU_DESC(0x17, 0x31, 0x0, 0x08301052),
+};
+
 static void fix_erratum_1386(struct cpuinfo_x86 *c)
 {
 	/*
@@ -811,7 +817,13 @@ static void fix_erratum_1386(struct cpuinfo_x86 *c)
 	 *
 	 * Affected parts all have no supervisor XSAVE states, meaning that
 	 * the XSAVEC instruction (which works fine) is equivalent.
+	 *
+	 * Clear the feature flag only on microcode revisions which
+	 * don't have the fix.
 	 */
+	if (x86_cpu_has_min_microcode_rev(erratum_1386_microcode))
+		return;
+
 	clear_cpu_cap(c, X86_FEATURE_XSAVES);
 }