cpu/x86/smm.h: Add function to return the SMM save state revision

Change-Id: I3e4450088adbb654050e7420956cf58ee1170a98
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44318
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/smm/smihandler.c

@@ -120,6 +120,14 @@ static inline void *smm_save_state(uintptr_t base, int arch_offset, int node)
 	return (void *)base;
 }
 
+/* This returns the SMM revision from the savestate of CPU0,
+   which is assumed to be the same for all CPU's. See the memory
+   map in smmhandler.S */
+uint32_t smm_revision(void)
+{
+	return *(uint32_t *)(SMM_BASE + SMM_ENTRY_OFFSET * 2 - SMM_REVISION_OFFSET_FROM_TOP);
+}
+
 bool smm_region_overlaps_handler(const struct region *r)
 {
 	const struct region r_smm = {SMM_BASE, SMM_DEFAULT_SIZE};

src/cpu/x86/smm/smm_module_handler.c

@@ -106,6 +106,13 @@ void *smm_get_save_state(int cpu)
 	return base;
 }
 
+uint32_t smm_revision(void)
+{
+	const uintptr_t save_state = (uintptr_t)(smm_get_save_state(0));
+
+	return *(uint32_t *)(save_state + smm_runtime->save_state_size - SMM_REVISION_OFFSET_FROM_TOP);
+}
+
 bool smm_region_overlaps_handler(const struct region *r)
 {
 	const struct region r_smm = {smm_runtime->smbase, smm_runtime->smm_size};

src/include/cpu/x86/smm.h

@@ -193,4 +193,9 @@ int smm_subregion(int sub, uintptr_t *start, size_t *size);
 /* Print the SMM memory layout on console. */
 void smm_list_regions(void);
 
+#define SMM_REVISION_OFFSET_FROM_TOP (0x8000 - 0x7efc)
+/* Return the SMM save state revision. The revision can be fetched from the smm savestate
+   which is always at the same offset downward from the top of the save state. */
+uint32_t smm_revision(void);
+
 #endif /* CPU_X86_SMM_H */