[v2,3/7] hw/acpi: extract standard table headers as a standalone structure

diff --git a/include/hw/acpi/acpi.h b/include/hw/acpi/acpi.h
index 635be7b..0e26f63 100644
--- a/include/hw/acpi/acpi.h
+++ b/include/hw/acpi/acpi.h
@@ -167,4 +167,15 @@  extern size_t acpi_tables_len;
 
 void acpi_table_add(const QemuOpts *opts, Error **errp);
 
+typedef struct acpi_table_std_header {
+    char sig[4];              /* ACPI signature (4 ASCII characters) */
+    uint32_t length;          /* Length of table, in bytes, including header */
+    uint8_t revision;         /* ACPI Specification minor version # */
+    uint8_t checksum;         /* To make sum of entire table == 0 */
+    char oem_id[6];           /* OEM identification */
+    char oem_table_id[8];     /* OEM table identification */
+    uint32_t oem_revision;    /* OEM revision number */
+    char asl_compiler_id[4];  /* ASL compiler vendor ID */
+    uint32_t asl_compiler_revision; /* ASL compiler revision number */
+} QEMU_PACKED AcpiTableStdHdr;
 #endif /* !QEMU_HW_ACPI_H */
diff --git a/hw/acpi/core.c b/hw/acpi/core.c
index 69cadb0..d348f81 100644
--- a/hw/acpi/core.c
+++ b/hw/acpi/core.c
@@ -31,21 +31,13 @@ 
 struct acpi_table_header {
     uint16_t _length;         /* our length, not actual part of the hdr */
                               /* allows easier parsing for fw_cfg clients */
-    char sig[4];              /* ACPI signature (4 ASCII characters) */
-    uint32_t length;          /* Length of table, in bytes, including header */
-    uint8_t revision;         /* ACPI Specification minor version # */
-    uint8_t checksum;         /* To make sum of entire table == 0 */
-    char oem_id[6];           /* OEM identification */
-    char oem_table_id[8];     /* OEM table identification */
-    uint32_t oem_revision;    /* OEM revision number */
-    char asl_compiler_id[4];  /* ASL compiler vendor ID */
-    uint32_t asl_compiler_revision; /* ASL compiler revision number */
+    AcpiTableStdHdr std;
 } QEMU_PACKED;
 
-#define ACPI_TABLE_HDR_SIZE sizeof(struct acpi_table_header)
-#define ACPI_TABLE_PFX_SIZE sizeof(uint16_t)  /* size of the extra prefix */
+/* size of the extra prefix */
+#define ACPI_TABLE_PFX_SIZE offsetof(struct acpi_table_header, std)
 
-static const char unsigned dfl_hdr[ACPI_TABLE_HDR_SIZE - ACPI_TABLE_PFX_SIZE] =
+static const char unsigned dfl_hdr[sizeof(AcpiTableStdHdr)] =
     "QEMU\0\0\0\0\1\0"       /* sig (4), len(4), revno (1), csum (1) */
     "QEMUQEQEMUQEMU\1\0\0\0" /* OEM id (6), table (8), revno (4) */
     "QEMU\1\0\0\0"           /* ASL compiler ID (4), version (4) */
@@ -105,6 +97,7 @@  static void acpi_table_install(const char unsigned *blob, size_t bloblen,
     size_t body_size, acpi_payload_size;
     struct acpi_table_header *ext_hdr;
     unsigned changed_fields;
+    AcpiTableStdHdr *std;
 
     /* Calculate where the ACPI table body starts within the blob, plus where
      * to copy the ACPI table header from.
@@ -177,46 +170,47 @@  static void acpi_table_install(const char unsigned *blob, size_t bloblen,
     changed_fields = 0;
     ext_hdr->_length = cpu_to_le16(acpi_payload_size);
 
+    std = &ext_hdr->std;
     if (hdrs->has_sig) {
-        strncpy(ext_hdr->sig, hdrs->sig, sizeof ext_hdr->sig);
+        strncpy(std->sig, hdrs->sig, sizeof std->sig);
         ++changed_fields;
     }
 
-    if (has_header && le32_to_cpu(ext_hdr->length) != acpi_payload_size) {
+    if (has_header && le32_to_cpu(std->length) != acpi_payload_size) {
         fprintf(stderr,
                 "warning: ACPI table has wrong length, header says "
                 "%" PRIu32 ", actual size %zu bytes\n",
-                le32_to_cpu(ext_hdr->length), acpi_payload_size);
+                le32_to_cpu(std->length), acpi_payload_size);
     }
-    ext_hdr->length = cpu_to_le32(acpi_payload_size);
+    std->length = cpu_to_le32(acpi_payload_size);
 
     if (hdrs->has_rev) {
-        ext_hdr->revision = hdrs->rev;
+        std->revision = hdrs->rev;
         ++changed_fields;
     }
 
-    ext_hdr->checksum = 0;
+    std->checksum = 0;
 
     if (hdrs->has_oem_id) {
-        strncpy(ext_hdr->oem_id, hdrs->oem_id, sizeof ext_hdr->oem_id);
+        strncpy(std->oem_id, hdrs->oem_id, sizeof std->oem_id);
         ++changed_fields;
     }
     if (hdrs->has_oem_table_id) {
-        strncpy(ext_hdr->oem_table_id, hdrs->oem_table_id,
-                sizeof ext_hdr->oem_table_id);
+        strncpy(std->oem_table_id, hdrs->oem_table_id,
+                sizeof std->oem_table_id);
         ++changed_fields;
     }
     if (hdrs->has_oem_rev) {
-        ext_hdr->oem_revision = cpu_to_le32(hdrs->oem_rev);
+        std->oem_revision = cpu_to_le32(hdrs->oem_rev);
         ++changed_fields;
     }
     if (hdrs->has_asl_compiler_id) {
-        strncpy(ext_hdr->asl_compiler_id, hdrs->asl_compiler_id,
-                sizeof ext_hdr->asl_compiler_id);
+        strncpy(std->asl_compiler_id, hdrs->asl_compiler_id,
+                sizeof std->asl_compiler_id);
         ++changed_fields;
     }
     if (hdrs->has_asl_compiler_rev) {
-        ext_hdr->asl_compiler_revision = cpu_to_le32(hdrs->asl_compiler_rev);
+        std->asl_compiler_revision = cpu_to_le32(hdrs->asl_compiler_rev);
         ++changed_fields;
     }
 
@@ -225,8 +219,8 @@  static void acpi_table_install(const char unsigned *blob, size_t bloblen,
     }
 
     /* recalculate checksum */
-    ext_hdr->checksum = acpi_checksum((const char unsigned *)ext_hdr +
-                                      ACPI_TABLE_PFX_SIZE, acpi_payload_size);
+    std->checksum = acpi_checksum((const char unsigned *)std,
+                                  acpi_payload_size);
 }
 
 void acpi_table_add(const QemuOpts *opts, Error **errp)