acpi: cpuhp: introduce 'Command data 2' field

No functional change in practice, patch only aims to properly
document (in spec and code) intended usage of the reserved space.

The new field is to be used for 2 purposes:
  - detection of modern CPU hotplug interface using
    CPHP_GET_NEXT_CPU_WITH_EVENT_CMD command.
    procedure will be described in follow up patch:
      "acpi: cpuhp: spec: add typical usecases"
  - for returning upper 32 bits of architecture specific CPU ID,
    for new CPHP_GET_CPU_ID_CMD command added by follow up patch:
      "acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command"

Change is backward compatible with 4.2 and older machines, as field was
unconditionally reserved and always returned 0x0 if modern CPU hotplug
interface was enabled.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1575896942-331151-8-git-send-email-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

docs/specs/acpi_cpu_hotplug.txt

@@ -44,7 +44,10 @@ keeps the current value.
 
 read access:
     offset:
-    [0x0-0x3] reserved
+    [0x0-0x3] Command data 2: (DWORD access)
+              if value last stored in 'Command field':
+                0: reads as 0x0
+                other values: reserved
     [0x4] CPU device status fields: (1 byte access)
         bits:
            0: Device is enabled and may be used by guest

hw/acpi/cpu.c

@@ -12,6 +12,7 @@
 #define ACPI_CPU_FLAGS_OFFSET_RW 4
 #define ACPI_CPU_CMD_OFFSET_WR 5
 #define ACPI_CPU_CMD_DATA_OFFSET_RW 8
+#define ACPI_CPU_CMD_DATA2_OFFSET_R 0
 
 enum {
     CPHP_GET_NEXT_CPU_WITH_EVENT_CMD = 0,
@@ -79,6 +80,16 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
         }
         trace_cpuhp_acpi_read_cmd_data(cpu_st->selector, val);
         break;
+    case ACPI_CPU_CMD_DATA2_OFFSET_R:
+        switch (cpu_st->command) {
+        case CPHP_GET_NEXT_CPU_WITH_EVENT_CMD:
+           val = 0;
+           break;
+        default:
+           break;
+        }
+        trace_cpuhp_acpi_read_cmd_data2(cpu_st->selector, val);
+        break;
     default:
         break;
     }

hw/acpi/trace-events

@@ -23,6 +23,7 @@ cpuhp_acpi_read_flags(uint32_t idx, uint8_t flags) "idx[0x%"PRIx32"] flags: 0x%"
 cpuhp_acpi_write_idx(uint32_t idx) "set active cpu idx: 0x%"PRIx32
 cpuhp_acpi_write_cmd(uint32_t idx, uint8_t cmd) "idx[0x%"PRIx32"] cmd: 0x%"PRIx8
 cpuhp_acpi_read_cmd_data(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
+cpuhp_acpi_read_cmd_data2(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
 cpuhp_acpi_cpu_has_events(uint32_t idx, bool ins, bool rm) "idx[0x%"PRIx32"] inserting: %d, removing: %d"
 cpuhp_acpi_clear_inserting_evt(uint32_t idx) "idx[0x%"PRIx32"]"
 cpuhp_acpi_clear_remove_evt(uint32_t idx) "idx[0x%"PRIx32"]"