Re: [PATCH v14 08/11] qapi/s390/cpu topology: change-topology monitor command

[Daniel P . Berrangé]

On Thu, Jan 05, 2023 at 03:53:10PM +0100, Pierre Morel wrote:
> The modification of the CPU attributes are done through a monitor
> commands.
> 
> It allows to move the core inside the topology tree to optimise
> the cache usage in the case the host's hypervizor previously
> moved the CPU.
> 
> The same command allows to modifiy the CPU attributes modifiers
> like polarization entitlement and the dedicated attribute to notify
> the guest if the host admin modified scheduling or dedication of a vCPU.
> 
> With this knowledge the guest has the possibility to optimize the
> usage of the vCPUs.
> 
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> ---
>  qapi/machine-target.json |  29 ++++++++
>  include/monitor/hmp.h    |   1 +
>  hw/s390x/cpu-topology.c  | 141 +++++++++++++++++++++++++++++++++++++++
>  hmp-commands.hx          |  16 +++++
>  4 files changed, 187 insertions(+)
> 
> diff --git a/qapi/machine-target.json b/qapi/machine-target.json
> index 2e267fa458..75b0aa254d 100644
> --- a/qapi/machine-target.json
> +++ b/qapi/machine-target.json
> @@ -342,3 +342,32 @@
>                     'TARGET_S390X',
>                     'TARGET_MIPS',
>                     'TARGET_LOONGARCH64' ] } }
> +
> +##
> +# @change-topology:
> +#
> +# @core: the vCPU ID to be moved
> +# @socket: the destination socket where to move the vCPU
> +# @book: the destination book where to move the vCPU
> +# @drawer: the destination drawer where to move the vCPU

This movement can be done while the guest OS is running ?
What happens to guest OS apps ? Every I know will read
topology once and assume it never changes at runtime.

What's the use case for wanting to re-arrange topology in
this manner ? It feels like its going to be a recipe for
hard to diagnose problems, as much code in libvirt and apps
above will assuming the vCPU IDs are assigned sequentially
starting from node=0,book=0,drawer=0,socket=0,core=0,
incrementing core, then incrementing socket, then
incrementing drawer, etc.

> +# @polarity: optional polarity, default is last polarity set by the guest
> +# @dedicated: optional, if the vCPU is dedicated to a real CPU
> +#
> +# Modifies the topology by moving the CPU inside the topology
> +# tree or by changing a modifier attribute of a CPU.
> +#
> +# Returns: Nothing on success, the reason on failure.
> +#
> +# Since: <next qemu stable release, eg. 1.0>
> +##
> +{ 'command': 'change-topology',

'set-cpu-topology'

> +  'data': {
> +      'core': 'int',
> +      'socket': 'int',
> +      'book': 'int',
> +      'drawer': 'int',
> +      '*polarity': 'int',
> +      '*dedicated': 'bool'
> +  },
> +  'if': { 'all': [ 'TARGET_S390X', 'CONFIG_KVM' ] }
> +}


With regards,
Daniel
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