Re: [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command

[Laszlo Ersek <lersek@redhat.com>]

On 10/09/19 15:22, Igor Mammedov wrote:
> Extend CPU hotplug interface to return architecture specific
> identifier for current CPU (in case of x86, it's APIC ID).
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> TODO:
>   * cripple it to behave old way on old machine types so that
>     new firmware started on new QEMU won't see a difference
>     when migrated to an old QEMU (i.e. QEMU that doesn't support
>     this command)
> ---
>  docs/specs/acpi_cpu_hotplug.txt | 10 +++++++++-
>  hw/acpi/cpu.c                   | 15 +++++++++++++++
>  hw/acpi/trace-events            |  1 +
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> index 43c5a193f0..0438678249 100644
> --- a/docs/specs/acpi_cpu_hotplug.txt
> +++ b/docs/specs/acpi_cpu_hotplug.txt
> @@ -32,7 +32,9 @@ Register block size:
>  
>  read access:
>      offset:
> -    [0x0-0x3] reserved
> +    [0x0-0x3] Command data 2: (DWORD access)
> +              upper 32 bit of 'Command data' if returned data value is 64 bit.
> +              in case of error or unsupported command reads is 0x0

How about

    [0x0] Command data 2: (DWORD access, little endian)
          If the "CPU selector" value last stored by the guest refers to
          an impossible CPU, then 0.
          Otherwise, if the "Command field" value last stored by the
          guest differs from 3, then 0.
          Otherwise, the most significant 32 bits of the selected CPU's
          architecture specific ID.

    [0x8] Command data: (DWORD access, little endian)
          If the "CPU selector" value last stored by the guest refers to
          an impossible CPU, then 0.
          Otherwise,
          - if the "Command field" value last stored by the guest is 0,
            then the selector of the currently selected CPU;
          - if the "Command field" value last stored by the guest is 3,
            then the least significant 32 bits of the selected CPU's
            architecture specific ID;
          - otherwise, 0.

>      [0x4] CPU device status fields: (1 byte access)
>          bits:
>             0: Device is enabled and may be used by guest
> @@ -87,6 +89,8 @@ write access:
>                2: stores value into OST status register, triggers
>                   ACPI_DEVICE_OST QMP event from QEMU to external applications
>                   with current values of OST event and status registers.
> +              3: OSPM reads architecture specific value identifying CPU
> +                 (x86: APIC ID)
>              other values: reserved
>  

Seems OK.

>  Selecting CPU device beyond possible range has no effect on platform:
> @@ -115,3 +119,7 @@ Typical usecases:
>       5.2 if 'Command data' register has not changed, there is not CPU
>           corresponding to iterator value and the last valid iterator value
>           equals to 'max_cpus' + 1
> +   - Get architecture specific id for a CPU
> +     1. pick a CPU to read from using 'CPU selector' register
> +     2. write 0x3 int0 'Command field' register
> +     3. read architecture specific id from 'Command data' register

Looks good, except for:

- typo: "int0"

- in step 3, we should reference 'Command data 2' as well.


In fact, in
<http://mid.mail-archive.com/2b10ca48-c734-4f41-9521-136c44060812@redhat.com>,
I wrote, for the "Get a cpu with pending event" use case:

> 1. Store 0x0 to the 'CPU selector' register.
> 2. Store 0x0 to the 'Command field' register.
> 3. Read the 'CPU device status fields' register.
> 4. If both bit#1 and bit#2 are clear in the value read, there is no
>    CPU with a pending event.
> 5. Otherwise, read the 'Command data' register. The value read is the
>    selector of the CPU with the pending event (which is already
>    selected).

and your steps #2 and #3, for getting the arch specific ID, can be
directly appended as steps 6. and 7.!


> diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
> index 87f30a31d7..701542d860 100644
> --- a/hw/acpi/cpu.c
> +++ b/hw/acpi/cpu.c
> @@ -12,11 +12,13 @@
>  #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_RW 0
>  
>  enum {
>      CPHP_GET_NEXT_CPU_WITH_EVENT_CMD = 0,
>      CPHP_OST_EVENT_CMD = 1,
>      CPHP_OST_STATUS_CMD = 2,
> +    CPHP_GET_CPU_ID_CMD = 3,
>      CPHP_CMD_MAX
>  };
>  
> @@ -74,11 +76,24 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
>          case CPHP_GET_NEXT_CPU_WITH_EVENT_CMD:
>             val = cpu_st->selector;
>             break;
> +        case CPHP_GET_CPU_ID_CMD:
> +           val = cpu_to_le64(cdev->arch_id) & 0xFFFFFFFF;
> +           break;
>          default:
>             break;
>          }
>          trace_cpuhp_acpi_read_cmd_data(cpu_st->selector, val);
>          break;
> +    case ACPI_CPU_CMD_DATA2_OFFSET_RW:
> +        switch (cpu_st->command) {
> +        case CPHP_GET_CPU_ID_CMD:
> +           val = cpu_to_le64(cdev->arch_id) >> 32;
> +           break;
> +        default:
> +           break;
> +        }
> +        trace_cpuhp_acpi_read_cmd_data2(cpu_st->selector, val);
> +        break;
>      default:
>          break;
>      }
> diff --git a/hw/acpi/trace-events b/hw/acpi/trace-events
> index 96b8273297..afbc77de1c 100644
> --- a/hw/acpi/trace-events
> +++ b/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"]"
> 

Looks plausible to me, thanks (discounting the TODO item).

Right now, I can't offer testing for patch#3 (I'm quite far from the
point where I'll be actually looking for a hotplugged CPU :) ), but
based on the docs patches #1 and #2, and my proposed updates, I can
rework my "possible CPU count detection" in OVMF.

Do I need to check in OVMF specifically whether the "modern" CPU hotplug
register block is available? Can you tell me what the oldest machine
types are that support the modern interface?

Hmm... Commit abd49bc2ed2f ("docs: update ACPI CPU hotplug spec with new
protocol", 2016-06-24) seems relevant. First released in v2.7.0. I think
I should detect whether this interface is available.

Can I use the following sequence to detect whether the interface is
available?

1. Store 0x0 to command register.
2. Store 0x0 to selector register.
3. Read 'command data' register.
4. If value read is 0, the interface is available.

(Because I assume that unmapped IO ports read as all-bits-one. Is that
right?)

BTW, can I dynamically detect support for the GET_CPU_ID command too?
I'm thinking, when I enumerate / count all possible CPUs, I can at once
fetch the arch IDs for all of them. If I only get zeros from the command
data registers, across all CPUs, in response to GET_CPU_ID, then the
command is not available.

Thanks
Laszlo