Re: [PATCH v14 09/11] qapi/s390/cpu topology: monitor query topology information

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Jan 05, 2023 at 03:53:11PM +0100, Pierre Morel wrote:
> Reporting the current topology informations to the admin through
> the QEMU monitor.
> 
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> ---
>  qapi/machine-target.json | 66 ++++++++++++++++++++++++++++++++++
>  include/monitor/hmp.h    |  1 +
>  hw/s390x/cpu-topology.c  | 76 ++++++++++++++++++++++++++++++++++++++++
>  hmp-commands-info.hx     | 16 +++++++++
>  4 files changed, 159 insertions(+)
> 
> diff --git a/qapi/machine-target.json b/qapi/machine-target.json
> index 75b0aa254d..927618a78f 100644
> --- a/qapi/machine-target.json
> +++ b/qapi/machine-target.json
> @@ -371,3 +371,69 @@
>    },
>    'if': { 'all': [ 'TARGET_S390X', 'CONFIG_KVM' ] }
>  }
> +
> +##
> +# @S390CpuTopology:
> +#
> +# CPU Topology information
> +#
> +# @drawer: the destination drawer where to move the vCPU
> +#
> +# @book: the destination book where to move the vCPU
> +#
> +# @socket: the destination socket where to move the vCPU
> +#
> +# @polarity: optional polarity, default is last polarity set by the guest
> +#
> +# @dedicated: optional, if the vCPU is dedicated to a real CPU
> +#
> +# @origin: offset of the first bit of the core mask
> +#
> +# @mask: mask of the cores sharing the same topology
> +#
> +# Since: 8.0
> +##
> +{ 'struct': 'S390CpuTopology',
> +  'data': {
> +      'drawer': 'int',
> +      'book': 'int',
> +      'socket': 'int',
> +      'polarity': 'int',
> +      'dedicated': 'bool',
> +      'origin': 'int',
> +      'mask': 'str'
> +  },
> +  'if': { 'all': [ 'TARGET_S390X', 'CONFIG_KVM' ] }
> +}
> +
> +##
> +# @query-topology:
> +#
> +# Return information about CPU Topology
> +#
> +# Returns a @CpuTopology instance describing the CPU Toplogy
> +# being currently used by QEMU.
> +#
> +# Since: 8.0
> +#
> +# Example:
> +#
> +# -> { "execute": "cpu-topology" }
> +# <- {"return": [
> +#     {
> +#         "drawer": 0,
> +#         "book": 0,
> +#         "socket": 0,
> +#         "polarity": 0,
> +#         "dedicated": true,
> +#         "origin": 0,
> +#         "mask": 0xc000000000000000,
> +#     },
> +#    ]
> +#   }
> +#
> +##
> +{ 'command': 'query-topology',
> +  'returns': ['S390CpuTopology'],
> +  'if': { 'all': [ 'TARGET_S390X', 'CONFIG_KVM' ] }
> +}

IIUC, you're using @mask as a way to compress the array returned
from query-topology, so that it doesn't have any repeated elements
with the same data. I guess I can understand that desire when the
core count can get very large, this can have a large saving.

The downside of using @mask, is that now you require the caller
to parse the string to turn it into a bitmask and expand the
data. Generally this is considered a bit of an anti-pattern in
QAPI design - we don't want callers to have to further parse
the data to extract information, we want to directly consumable
from the parsed JSON doc.

We already have 'query-cpus-fast' wich returns one entry for
each CPU. In fact why do we need to add query-topology at all.
Can't we just add book-id / drawer-id / polarity / dedicated
to the query-cpus-fast result ?

With regards,
Daniel
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