KVM: SVM: Add host SEV-ES save area structure into VMCB via a union

Incorporate the _host_ SEV-ES save area into the VMCB as a union with the
legacy save area.  The SEV-ES variant used to save/load host state is
larger than the legacy save area, but resides at the same offset.  Prefix
the field with "host" to make it as obvious as possible that the SEV-ES
variant in the VMCB is only ever used for host state.  Guest state for
SEV-ES VMs is stored in a completely separate page (VMSA), albeit with
the same layout as the host state.

Add a compile-time assert to ensure the VMCB layout is correct, i.e. that
KVM's layout matches the architectural definitions.

No functional change intended.

Link: https://lore.kernel.org/r/20240802204511.352017-3-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/include/asm/svm.h

@@ -516,6 +516,20 @@ struct ghcb {
 	u32 ghcb_usage;
 } __packed;
 
+struct vmcb {
+	struct vmcb_control_area control;
+	union {
+		struct vmcb_save_area save;
+
+		/*
+		 * For SEV-ES VMs, the save area in the VMCB is used only to
+		 * save/load host state.  Guest state resides in a separate
+		 * page, the aptly named VM Save Area (VMSA), that is encrypted
+		 * with the guest's private key.
+		 */
+		struct sev_es_save_area host_sev_es_save;
+	};
+} __packed;
 
 #define EXPECTED_VMCB_SAVE_AREA_SIZE		744
 #define EXPECTED_GHCB_SAVE_AREA_SIZE		1032
@@ -532,6 +546,7 @@ static inline void __unused_size_checks(void)
 	BUILD_BUG_ON(sizeof(struct ghcb_save_area)	!= EXPECTED_GHCB_SAVE_AREA_SIZE);
 	BUILD_BUG_ON(sizeof(struct sev_es_save_area)	!= EXPECTED_SEV_ES_SAVE_AREA_SIZE);
 	BUILD_BUG_ON(sizeof(struct vmcb_control_area)	!= EXPECTED_VMCB_CONTROL_AREA_SIZE);
+	BUILD_BUG_ON(offsetof(struct vmcb, save)	!= EXPECTED_VMCB_CONTROL_AREA_SIZE);
 	BUILD_BUG_ON(sizeof(struct ghcb)		!= EXPECTED_GHCB_SIZE);
 
 	/* Check offsets of reserved fields */
@@ -568,11 +583,6 @@ static inline void __unused_size_checks(void)
 	BUILD_BUG_RESERVED_OFFSET(ghcb, 0xff0);
 }
 
-struct vmcb {
-	struct vmcb_control_area control;
-	struct vmcb_save_area save;
-} __packed;
-
 #define SVM_CPUID_FUNC 0x8000000a
 
 #define SVM_SELECTOR_S_SHIFT 4