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

[Gavin Shan <gshan@redhat.com>]

Hi Salil,

On 10/1/25 11:01 AM, salil.mehta@opnsrc.net wrote:
> 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>
> ---
>   hw/core/machine-smp.c | 24 +++++++-----
>   include/hw/boards.h   |  2 +
>   qapi/machine.json     |  3 ++
>   qemu-options.hx       | 86 +++++++++++++++++++++++++++++++++----------
>   system/vl.c           |  3 ++
>   5 files changed, 89 insertions(+), 29 deletions(-)
> 
> diff --git a/hw/core/machine-smp.c b/hw/core/machine-smp.c
> index 0be0ac044c..c1a09fdc3f 100644
> --- a/hw/core/machine-smp.c
> +++ b/hw/core/machine-smp.c
> @@ -87,6 +87,7 @@ void machine_parse_smp_config(MachineState *ms,
>   {
>       MachineClass *mc = MACHINE_GET_CLASS(ms);
>       unsigned cpus    = config->has_cpus ? config->cpus : 0;
> +    unsigned disabledcpus = config->has_disabledcpus ? config->disabledcpus : 0;
>       unsigned drawers = config->has_drawers ? config->drawers : 0;
>       unsigned books   = config->has_books ? config->books : 0;
>       unsigned sockets = config->has_sockets ? config->sockets : 0;
> @@ -166,8 +167,13 @@ void machine_parse_smp_config(MachineState *ms,
>           sockets = sockets > 0 ? sockets : 1;
>           cores = cores > 0 ? cores : 1;
>           threads = threads > 0 ? threads : 1;
> +
> +        maxcpus = drawers * books * sockets * dies * clusters *
> +                    modules * cores * threads;
> +        cpus = maxcpus - disabledcpus;
>       } else {
> -        maxcpus = maxcpus > 0 ? maxcpus : cpus;
> +        maxcpus = maxcpus > 0 ? maxcpus : cpus + disabledcpus;
> +        cpus = cpus > 0 ? cpus : maxcpus - disabledcpus;
>   
>           if (mc->smp_props.prefer_sockets) {
>               /* prefer sockets over cores before 6.2 */
> @@ -207,12 +213,8 @@ void machine_parse_smp_config(MachineState *ms,
>           }
>       }
>   
> -    total_cpus = drawers * books * sockets * dies *
> -                 clusters * modules * cores * threads;
> -    maxcpus = maxcpus > 0 ? maxcpus : total_cpus;
> -    cpus = cpus > 0 ? cpus : maxcpus;
> -
>       ms->smp.cpus = cpus;
> +    ms->smp.disabledcpus = disabledcpus;
>       ms->smp.drawers = drawers;
>       ms->smp.books = books;
>       ms->smp.sockets = sockets;
> @@ -226,6 +228,8 @@ void machine_parse_smp_config(MachineState *ms,
>       mc->smp_props.has_clusters = config->has_clusters;
>   
>       /* sanity-check of the computed topology */
> +    total_cpus = maxcpus = drawers * books * sockets * dies * clusters *
> +                modules * cores * threads;
>       if (total_cpus != maxcpus) {
>           g_autofree char *topo_msg = cpu_hierarchy_to_string(ms);
>           error_setg(errp, "Invalid CPU topology: "

It's enforced that total_cpus is equal to maxcpus. The followup check
"if (total_cpus != maxcpus)" becomes unnecessary. Also, does this makes
"-smp maxcpus=x" unnecessary?

Thanks,
Gavin

> @@ -235,12 +239,12 @@ void machine_parse_smp_config(MachineState *ms,
>           return;
>       }
>   
> -    if (maxcpus < cpus) {
> +    if (maxcpus < (cpus + disabledcpus)) {
>           g_autofree char *topo_msg = cpu_hierarchy_to_string(ms);
>           error_setg(errp, "Invalid CPU topology: "
> -                   "maxcpus must be equal to or greater than smp: "
> -                   "%s == maxcpus (%u) < smp_cpus (%u)",
> -                   topo_msg, maxcpus, cpus);
> +                   "maxcpus must be equal to or greater than smp[+disabledcpus]:"
> +                   "%s == maxcpus (%u) < smp_cpus (%u) [+ offline cpus (%u)]",
> +                   topo_msg, maxcpus, cpus, disabledcpus);
>           return;
>       }
>   
> diff --git a/include/hw/boards.h b/include/hw/boards.h
> index f94713e6e2..2b182d7817 100644
> --- a/include/hw/boards.h
> +++ b/include/hw/boards.h
> @@ -361,6 +361,7 @@ typedef struct DeviceMemoryState {
>   /**
>    * CpuTopology:
>    * @cpus: the number of present logical processors on the machine
> + * @disabledcpus: the number additional present but admin disabled cpus
>    * @drawers: the number of drawers on the machine
>    * @books: the number of books in one drawer
>    * @sockets: the number of sockets in one book
> @@ -373,6 +374,7 @@ typedef struct DeviceMemoryState {
>    */
>   typedef struct CpuTopology {
>       unsigned int cpus;
> +    unsigned int disabledcpus;
>       unsigned int drawers;
>       unsigned int books;
>       unsigned int sockets;
> 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',
> 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
> +
>       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,