Re: [PATCH RFC V6 02/24] hw/core, qemu-options.hx: Introduce 'disabledcpus' SMP parameter

[Markus Armbruster <armbru@redhat.com>]

salil.mehta@opnsrc.net writes:

> From: Salil Mehta <salil.mehta@huawei.com>
>
> Add support for a new SMP configuration parameter, 'disabledcpus', which
> specifies the number of additional CPUs that are present in the virtual
> machine but administratively disabled at boot. These CPUs are visible in
> firmware (e.g. ACPI tables) yet unavailable to the guest until explicitly
> enabled via QMP/HMP, or via the 'device_set' API (introduced in later
> patches).
>
> This feature is intended for architectures that lack native CPU hotplug
> support but can change the administrative power state of present CPUs.
> It allows simulating CPU hot-add–like scenarios while all CPUs remain
> physically present in the topology at boot time.
>
> Note: ARM is the first architecture to support this concept.
>
> Changes include:
>  - Extend CpuTopology with a 'disabledcpus' field.
>  - Update machine_parse_smp_config() to account for disabled CPUs when
>    computing 'cpus' and 'maxcpus'.
>  - Update SMPConfiguration in QAPI to accept 'disabledcpus'.
>  - Extend -smp option documentation to describe 'disabledcpus' usage and
>    behavior.
>
> Signed-off-by: Salil Mehta <salil.mehta@huawei.com>

[...]

> diff --git a/qapi/machine.json b/qapi/machine.json
> index 038eab281c..e45740da33 100644
> --- a/qapi/machine.json
> +++ b/qapi/machine.json
> @@ -1634,6 +1634,8 @@
>  #
>  # @cpus: number of virtual CPUs in the virtual machine
>  #
> +# @disabledcpus: number of additional present but disabled(or offline) CPUs
> +#
>  # @maxcpus: maximum number of hotpluggable virtual CPUs in the virtual
>  #     machine
>  #
> @@ -1657,6 +1659,7 @@
>  ##
>  { 'struct': 'SMPConfiguration', 'data': {
>       '*cpus': 'int',
> +     '*disabledcpus': 'int',
>       '*drawers': 'int',
>       '*books': 'int',
>       '*sockets': 'int',

We prefer words-with-dashes to wordsruntogether in QAPI/QMP, for
readability: disabled-cpus.

Missing here even before the patch: how these values are related.  That
information appears to be buried in machine_parse_smp_config(), which is
160 lines long and with too many conditionals for me to make sense of
now.

> diff --git a/qemu-options.hx b/qemu-options.hx
> index ab23f14d21..83ccde341b 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -326,12 +326,15 @@ SRST
>  ERST
>  
>  DEF("smp", HAS_ARG, QEMU_OPTION_smp,
> -    "-smp [[cpus=]n][,maxcpus=maxcpus][,drawers=drawers][,books=books][,sockets=sockets]\n"
> -    "               [,dies=dies][,clusters=clusters][,modules=modules][,cores=cores]\n"
> -    "               [,threads=threads]\n"
> -    "                set the number of initial CPUs to 'n' [default=1]\n"
> -    "                maxcpus= maximum number of total CPUs, including\n"
> -    "                offline CPUs for hotplug, etc\n"
> +    "-smp [[cpus=]n][,disabledcpus=disabledcpus][,maxcpus=maxcpus][,drawers=drawers][,books=books]\n"
> +    "               [,sockets=sockets][,dies=dies][,clusters=clusters][,modules=modules]\n"
> +    "               [,cores=cores][,threads=threads]\n"
> +    "                set the initial number of CPUs present and\n"
> +    "                  administratively enabled at boot time to 'n' [default=1]\n"
> +    "                disabledcpus= number of present but administratively\n"
> +    "                  disabled CPUs (unavailable to the guest at boot)\n"
> +    "                maxcpus= maximum total CPUs (present + hotpluggable)\n"
> +    "                  on machines without CPU hotplug, defaults to n + disabledcpus\n"
>      "                drawers= number of drawers on the machine board\n"
>      "                books= number of books in one drawer\n"
>      "                sockets= number of sockets in one book\n"
> @@ -351,22 +354,49 @@ DEF("smp", HAS_ARG, QEMU_OPTION_smp,
>      "      For a particular machine type board, an expected CPU topology hierarchy\n"
>      "      can be defined through the supported sub-option. Unsupported parameters\n"
>      "      can also be provided in addition to the sub-option, but their values\n"
> -    "      must be set as 1 in the purpose of correct parsing.\n",
> +    "      must be set as 1 in the purpose of correct parsing.\n"
> +    "                                                          \n"
> +    "      Administratively disabled CPUs: Some machine types do not support vCPU\n"
> +    "      hotplug but their CPUs can be marked disabled (powered off) and kept\n"
> +    "      unavailable to the guest. Later, such CPUs can be enabled via QMP/HMP\n"
> +    "      (e.g., 'device_set ... admin-state=enable'). This is similar to hotplug,\n"
> +    "      except all disabled CPUs are already present at boot. Useful on\n"
> +    "      architectures that lack architectural CPU hotplug.\n",
>      QEMU_ARCH_ALL)
>  SRST
> -``-smp [[cpus=]n][,maxcpus=maxcpus][,drawers=drawers][,books=books][,sockets=sockets][,dies=dies][,clusters=clusters][,modules=modules][,cores=cores][,threads=threads]``
> -    Simulate a SMP system with '\ ``n``\ ' CPUs initially present on
> -    the machine type board. On boards supporting CPU hotplug, the optional
> -    '\ ``maxcpus``\ ' parameter can be set to enable further CPUs to be
> -    added at runtime. When both parameters are omitted, the maximum number
> +``-smp [[cpus=]n][,disabledcpus=disabledcpus][,maxcpus=maxcpus][,drawers=drawers][,books=books][,sockets=sockets][,dies=dies][,clusters=clusters][,modules=modules][,cores=cores][,threads=threads]``
> +    Simulate a SMP system with '\ ``n``\ ' CPUs initially present & enabled on
> +    the machine type board. Furthermore, on architectures that support changing
> +    the administrative power state of CPUs, optional '\ ``disabledcpus``\ '
> +    parameter specifies *additional* CPUs that are present in firmware (e.g.,
> +    ACPI) but are administratively disabled (i.e., not usable by the guest at
> +    boot time).
> +
> +    This is different from CPU hotplug where additional CPUs are not even
> +    present in the system description. Administratively disabled CPUs appear in
> +    ACPI tables i.e. are provisioned, but cannot be used until explicitly
> +    enabled via QMP/HMP or the deviceset API.
> +
> +    On boards supporting CPU hotplug, the optional '\ ``maxcpus``\ ' parameter
> +    can be set to enable further CPUs to be added at runtime. When both
> +    '\ ``n``\ ' & '\ ``maxcpus``\ ' parameters are omitted, the maximum number
>      of CPUs will be calculated from the provided topology members and the
> -    initial CPU count will match the maximum number. When only one of them
> -    is given then the omitted one will be set to its counterpart's value.
> -    Both parameters may be specified, but the maximum number of CPUs must
> -    be equal to or greater than the initial CPU count. Product of the
> -    CPU topology hierarchy must be equal to the maximum number of CPUs.
> -    Both parameters are subject to an upper limit that is determined by
> -    the specific machine type chosen.
> +    initial CPU count will match the maximum number. When only one of them is
> +    given then the omitted one will be set to its counterpart's value. Both
> +    parameters may be specified, but the maximum number of CPUs must be equal
> +    to or greater than the initial CPU count. Product of the CPU topology
> +    hierarchy must be equal to the maximum number of CPUs. Both parameters are
> +    subject to an upper limit that is determined by the specific machine type
> +    chosen. Boards that support administratively disabled CPUs but do *not*
> +    support CPU hotplug derive the maximum number of CPUs implicitly:
> +    '\ ``maxcpus``\ ' is treated as '\ ``n + disabledcpus``\ ' (the total CPUs
> +    present in firmware). If '\ ``maxcpus``\ ' is provided, it must equal
> +    '\ ``n + disabledcpus``\ '. The topology product must equal this derived
> +    maximum as well.
> +
> +    Note: Administratively disabled CPUs will appear to the guest as
> +    unavailable, and any attempt to bring them online must go through QMP/HMP
> +    commands like 'device_set'.
>  
>      To control reporting of CPU topology information, values of the topology
>      parameters can be specified. Machines may only support a subset of the
> @@ -425,6 +455,24 @@ SRST
>  
>          -smp 2
>  
> +    Examples using 'disabledcpus':
> +
> +    For a board without CPU hotplug, enable 4 CPUs at boot and provision
> +    2 additional administratively disabled CPUs (maximum is derived
> +    implicitly as 6 = 4 + 2):
> +
> +    ::
> +
> +        -smp cpus=4,disabledcpus=2
> +
> +    For a board that supports CPU hotplug and 'disabledcpus', enable 4 CPUs
> +    at boot, provision 2 administratively disabled CPUs, and allow hotplug of
> +    2 more CPUs (for a maximum of 8):
> +
> +    ::
> +
> +        -smp cpus=4,disabledcpus=2,maxcpus=8
> +

So, maxcpus = cpus + disabledcpus?

Is drawers * books * sockets * dies * clusters * modules * cores *
threads equal or greater than maxcpus?

>      Note: The cluster topology will only be generated in ACPI and exposed
>      to guest if it's explicitly specified in -smp.
>  ERST
> diff --git a/system/vl.c b/system/vl.c
> index 3b7057e6c6..2f0fd21a1f 100644
> --- a/system/vl.c
> +++ b/system/vl.c
> @@ -736,6 +736,9 @@ static QemuOptsList qemu_smp_opts = {
>          {
>              .name = "cpus",
>              .type = QEMU_OPT_NUMBER,
> +        }, {
> +            .name = "disabledcpus",
> +            .type = QEMU_OPT_NUMBER,
>          }, {
>              .name = "drawers",
>              .type = QEMU_OPT_NUMBER,