arm64: efi: Use SMBIOS processor version to key off Ampere quirk

Instead of using the SMBIOS type 1 record 'family' field, which is often
modified by OEMs, use the type 4 'processor ID' and 'processor version'
fields, which are set to a small set of probe-able values on all known
Ampere EFI systems in the field.

Fixes: 550b33cfd4 ("arm64: efi: Force the use of ...")
Tested-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/arm64.c

@@ -16,20 +16,43 @@
 
 static bool system_needs_vamap(void)
 {
-	const u8 *type1_family = efi_get_smbios_string(1, family);
+	const struct efi_smbios_type4_record *record;
+	const u32 __aligned(1) *socid;
+	const u8 *version;
 
 	/*
 	 * Ampere eMAG, Altra, and Altra Max machines crash in SetTime() if
-	 * SetVirtualAddressMap() has not been called prior.
+	 * SetVirtualAddressMap() has not been called prior. Most Altra systems
+	 * can be identified by the SMCCC soc ID, which is conveniently exposed
+	 * via the type 4 SMBIOS records. Otherwise, test the processor version
+	 * field. eMAG systems all appear to have the processor version field
+	 * set to "eMAG".
 	 */
-	if (!type1_family || (
-	    strcmp(type1_family, "eMAG") &&
-	    strcmp(type1_family, "Altra") &&
-	    strcmp(type1_family, "Altra Max")))
+	record = (struct efi_smbios_type4_record *)efi_get_smbios_record(4);
+	if (!record)
 		return false;
 
-	efi_warn("Working around broken SetVirtualAddressMap()\n");
-	return true;
+	socid = (u32 *)record->processor_id;
+	switch (*socid & 0xffff000f) {
+		static char const altra[] = "Ampere(TM) Altra(TM) Processor";
+		static char const emag[] = "eMAG";
+
+	default:
+		version = efi_get_smbios_string(&record->header, 4,
+						processor_version);
+		if (!version || (strncmp(version, altra, sizeof(altra) - 1) &&
+				 strncmp(version, emag, sizeof(emag) - 1)))
+			break;
+
+		fallthrough;
+
+	case 0x0a160001:	// Altra
+	case 0x0a160002:	// Altra Max
+		efi_warn("Working around broken SetVirtualAddressMap()\n");
+		return true;
+	}
+
+	return false;
 }
 
 efi_status_t check_platform_features(void)

drivers/firmware/efi/libstub/efistub.h

@@ -1074,6 +1074,8 @@ struct efi_smbios_record {
 	u16	handle;
 };
 
+const struct efi_smbios_record *efi_get_smbios_record(u8 type);
+
 struct efi_smbios_type1_record {
 	struct efi_smbios_record	header;
 
@@ -1087,14 +1089,47 @@ struct efi_smbios_type1_record {
 	u8				family;
 };
 
-#define efi_get_smbios_string(__type, __name) ({			\
+struct efi_smbios_type4_record {
+	struct efi_smbios_record	header;
+
+	u8				socket;
+	u8				processor_type;
+	u8				processor_family;
+	u8				processor_manufacturer;
+	u8				processor_id[8];
+	u8				processor_version;
+	u8				voltage;
+	u16				external_clock;
+	u16				max_speed;
+	u16				current_speed;
+	u8				status;
+	u8				processor_upgrade;
+	u16				l1_cache_handle;
+	u16				l2_cache_handle;
+	u16				l3_cache_handle;
+	u8				serial_number;
+	u8				asset_tag;
+	u8				part_number;
+	u8				core_count;
+	u8				enabled_core_count;
+	u8				thread_count;
+	u16				processor_characteristics;
+	u16				processor_family2;
+	u16				core_count2;
+	u16				enabled_core_count2;
+	u16				thread_count2;
+	u16				thread_enabled;
+};
+
+#define efi_get_smbios_string(__record, __type, __name) ({		\
 	int size = sizeof(struct efi_smbios_type ## __type ## _record);	\
 	int off = offsetof(struct efi_smbios_type ## __type ## _record,	\
 			   __name);					\
-	__efi_get_smbios_string(__type, off, size);			\
+	__efi_get_smbios_string((__record), __type, off, size);		\
 })
 
-const u8 *__efi_get_smbios_string(u8 type, int offset, int recsize);
+const u8 *__efi_get_smbios_string(const struct efi_smbios_record *record,
+				  u8 type, int offset, int recsize);
 
 void efi_remap_image(unsigned long image_base, unsigned alloc_size,
 		     unsigned long code_size);

drivers/firmware/efi/libstub/smbios.c

@@ -22,19 +22,28 @@ struct efi_smbios_protocol {
 	u8 minor_version;
 };
 
-const u8 *__efi_get_smbios_string(u8 type, int offset, int recsize)
+const struct efi_smbios_record *efi_get_smbios_record(u8 type)
 {
 	struct efi_smbios_record *record;
 	efi_smbios_protocol_t *smbios;
 	efi_status_t status;
 	u16 handle = 0xfffe;
-	const u8 *strtable;
 
 	status = efi_bs_call(locate_protocol, &EFI_SMBIOS_PROTOCOL_GUID, NULL,
 			     (void **)&smbios) ?:
 		 efi_call_proto(smbios, get_next, &handle, &type, &record, NULL);
 	if (status != EFI_SUCCESS)
 		return NULL;
+	return record;
+}
+
+const u8 *__efi_get_smbios_string(const struct efi_smbios_record *record,
+				  u8 type, int offset, int recsize)
+{
+	const u8 *strtable;
+
+	if (!record)
+		return NULL;
 
 	strtable = (u8 *)record + record->length;
 	for (int i = 1; i < ((u8 *)record)[offset]; i++) {