Re: [PATCH v1 3/7] util: Introduce ThreadContext user-creatable object

[Markus Armbruster <armbru@redhat.com>]

David Hildenbrand <david@redhat.com> writes:

> Setting the CPU affinity of QEMU threads is a bit problematic, because
> QEMU doesn't always have permissions to set the CPU affinity itself,
> for example, with seccomp after initialized by QEMU:
>     -sandbox enable=on,resourcecontrol=deny
>
> While upper layers are already aware how to handl;e CPU affinities for

Typo in handle.

> long-lived threads like iothreads or vcpu threads, especially short-lived
> threads, as used for memory-backend preallocation, are more involved to
> handle. These threads are created on demand and upper layers are not even
> able to identify and configure them.
>
> Introduce the concept of a ThreadContext, that is essentially a thread
> used for creating new threads. All threads created via that context
> thread inherit the configured CPU affinity. Consequently, it's
> sufficient to create a ThreadContext and configure it once, and have all
> threads created via that ThreadContext inherit the same CPU affinity.
>
> The CPU affinity of a ThreadContext can be configured two ways:
>
> (1) Obtaining the thread id via the "thread-id" property and setting the
>     CPU affinity manually.
>
> (2) Setting the "cpu-affinity" property and letting QEMU try set the
>     CPU affinity itself. This will fail if QEMU doesn't have permissions
>     to do so anymore after seccomp was initialized.

Could you provide usage examples?

>
> A ThreadContext can be reused, simply be reconfiguring the CPU affinity.
>
> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index 80dd419b39..4775a333ed 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -830,6 +830,20 @@
>              'reduced-phys-bits': 'uint32',
>              '*kernel-hashes': 'bool' } }
>  
> +##
> +# @ThreadContextProperties:
> +#
> +# Properties for thread context objects.
> +#
> +# @cpu-affinity: the CPU affinity for all threads created in the thread
> +#                context (default: QEMU main thread affinity)
> +#
> +# Since: 7.2
> +##
> +{ 'struct': 'ThreadContextProperties',
> +  'data': { '*cpu-affinity': ['uint16'] } }

I understand this is a list of affinities.  What I poor ignorant me
doesn't understand is the meaning of the list index.  Or in other words,
the list maps some range [0:N] to affinities, but what are the numbers
being mapped there?

> +
> +
>  ##
>  # @ObjectType:
>  #
> @@ -882,6 +896,7 @@
>      { 'name': 'secret_keyring',
>        'if': 'CONFIG_SECRET_KEYRING' },
>      'sev-guest',
> +    'thread-context',
>      's390-pv-guest',
>      'throttle-group',
>      'tls-creds-anon',
> @@ -948,6 +963,7 @@
>        'secret_keyring':             { 'type': 'SecretKeyringProperties',
>                                        'if': 'CONFIG_SECRET_KEYRING' },
>        'sev-guest':                  'SevGuestProperties',
> +      'thread-context':             'ThreadContextProperties',
>        'throttle-group':             'ThrottleGroupProperties',
>        'tls-creds-anon':             'TlsCredsAnonProperties',
>        'tls-creds-psk':              'TlsCredsPskProperties',

[...]