hw/acpi: Provide function acpi_ghes_present()

Allow code elsewhere in the system to check whether the ACPI GHES
table is present, so it can determine whether it is OK to try to
record an error by calling acpi_ghes_record_errors().

(We don't need to migrate the new 'present' field in AcpiGhesState,
because it is set once at system initialization and doesn't change.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Dongjiu Geng <gengdongjiu1@gmail.com>
Message-id: 20210603171259.27962-3-peter.maydell@linaro.org

hw/acpi/ghes-stub.c

@@ -15,3 +15,8 @@ int acpi_ghes_record_errors(uint8_t source_id, uint64_t physical_address)
 {
     return -1;
 }
+
+bool acpi_ghes_present(void)
+{
+    return false;
+}

hw/acpi/ghes.c

@@ -386,6 +386,8 @@ void acpi_ghes_add_fw_cfg(AcpiGhesState *ags, FWCfgState *s,
     /* Create a read-write fw_cfg file for Address */
     fw_cfg_add_file_callback(s, ACPI_GHES_DATA_ADDR_FW_CFG_FILE, NULL, NULL,
         NULL, &(ags->ghes_addr_le), sizeof(ags->ghes_addr_le), false);
+
+    ags->present = true;
 }
 
 int acpi_ghes_record_errors(uint8_t source_id, uint64_t physical_address)
@@ -443,3 +445,18 @@ int acpi_ghes_record_errors(uint8_t source_id, uint64_t physical_address)
 
     return ret;
 }
+
+bool acpi_ghes_present(void)
+{
+    AcpiGedState *acpi_ged_state;
+    AcpiGhesState *ags;
+
+    acpi_ged_state = ACPI_GED(object_resolve_path_type("", TYPE_ACPI_GED,
+                                                       NULL));
+
+    if (!acpi_ged_state) {
+        return false;
+    }
+    ags = &acpi_ged_state->ghes_state;
+    return ags->present;
+}