Re: [PATCH 07/12] machine: Support extra board CPUs outside -smp

[Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>]

Phil, Pierrick,

This patch seems to dwell around your turf.  Did you think about how to
instantiate different CPUs that might have different SMP topologies in
the context of single binary/split accel work?


Thanks,
Daniel

On 10/15/2026 12:17 AM, Portia Stephens wrote:
> From: Joel Stanley <joel@jms.id.au>
> 
> Boards may instantiate one or more CPUs that are not part of the
> user's -smp layout, for example an a management or boot core of a
> different CPU model. As TCG sizes its vCPU context from smp.max_cpus
> during init, creating more CPUState objects than smp.max_cpus trips the
> assert in tcg_register_thread().
> 
> smp.max_cpus is finalized before the accelerator is initialised and the
> board's init() runs later still, so a board's initialise cannot influence
> the count TCG uses.
> 
> Add a MachineClass::extra_cpus that boards set at class init time to
> declare these additional CPUs. TCG can then add it to max_threads. The
> field defaults to 0, so existing machines are unaffected.
> 
> Signed-off-by: Joel Stanley <joel@jms.id.au>
> Signed-off-by: Portia Stephens <portias@oss.tenstorrent.com>
> ---
>   accel/tcg/tcg-all.c      | 2 +-
>   include/hw/core/boards.h | 4 ++++
>   2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/accel/tcg/tcg-all.c b/accel/tcg/tcg-all.c
> index 767e880555..655609ebbb 100644
> --- a/accel/tcg/tcg-all.c
> +++ b/accel/tcg/tcg-all.c
> @@ -126,7 +126,7 @@ static int tcg_init_machine(AccelState *as, MachineState *ms)
>            */
>           if (mttcg_supported && !icount_enabled()) {
>               s->mttcg_enabled = ON_OFF_AUTO_ON;
> -            max_threads = ms->smp.max_cpus;
> +            max_threads = ms->smp.max_cpus + MACHINE_GET_CLASS(ms)->extra_cpus;
>           } else {
>               s->mttcg_enabled = ON_OFF_AUTO_OFF;
>           }
> diff --git a/include/hw/core/boards.h b/include/hw/core/boards.h
> index d0e4ad9820..575c6bcb31 100644
> --- a/include/hw/core/boards.h
> +++ b/include/hw/core/boards.h
> @@ -189,6 +189,9 @@ typedef struct {
>    * @max_cpus: maximum number of CPUs supported. Default: 1
>    * @min_cpus: minimum number of CPUs supported. Default: 1
>    * @default_cpus: number of CPUs instantiated if none are specified. Default: 1
> + * @extra_cpus: number of additional CPUs the board always instantiates beyond
> + *    the -smp topology (e.g. an always-present management core). Accelerators
> + *    add this to the per-vCPU state they allocate. Default: 0
>    * @is_default:
>    *    If true QEMU will use this machine by default if no '-M' option is given.
>    * @get_hotplug_handler: this function is called during bus-less
> @@ -287,6 +290,7 @@ struct MachineClass {
>       int max_cpus;
>       int min_cpus;
>       int default_cpus;
> +    int extra_cpus;
>       unsigned int no_serial:1,
>           no_parallel:1,
>           no_floppy:1,