Re: [PATCH v12] qapi: introduce 'query-x86-cpuid' QMP command.

[Markus Armbruster <armbru@redhat.com>]

I'm afraid this needs a rebase now.  Reviewing anyway.

Valeriy Vdovin <valeriy.vdovin@virtuozzo.com> writes:

> Introducing new QMP command 'query-x86-cpuid'. This command can be used to
> get virtualized cpu model info generated by QEMU during VM initialization in
> the form of cpuid representation.
>
> Diving into more details about virtual CPU generation: QEMU first parses '-cpu'
> command line option. From there it takes the name of the model as the basis for
> feature set of the new virtual CPU. After that it uses trailing '-cpu' options,
> that state if additional cpu features should be present on the virtual CPU or
> excluded from it (tokens '+'/'-' or '=on'/'=off').
> After that QEMU checks if the host's cpu can actually support the derived
> feature set and applies host limitations to it.
> After this initialization procedure, virtual CPU has it's model and
> vendor names, and a working feature set and is ready for identification
> instructions such as CPUID.
>
> To learn exactly how virtual CPU is presented to the guest machine via CPUID
> instruction, new QMP command can be used. By calling 'query-x86-cpuid'
> command, one can get a full listing of all CPUID leaves with subleaves which are
> supported by the initialized virtual CPU.
>
> Other than debug, the command is useful in cases when we would like to
> utilize QEMU's virtual CPU initialization routines and put the retrieved
> values into kernel CPUID overriding mechanics for more precise control
> over how various processes perceive its underlying hardware with
> container processes as a good example.
>
> The command is specific to x86. It is currenly only implemented for KVM acceleator.
>
> Output format:
> The output is a plain list of leaf/subleaf argument combinations, that
> return 4 words in registers EAX, EBX, ECX, EDX.
>
> Use example:
> qmp_request: {
>   "execute": "query-x86-cpuid"
> }
>
> qmp_response: {
>   "return": [
>     {
>       "eax": 1073741825,
>       "edx": 77,
>       "in-eax": 1073741824,
>       "ecx": 1447775574,
>       "ebx": 1263359563
>     },
>     {
>       "eax": 16777339,
>       "edx": 0,
>       "in-eax": 1073741825,
>       "ecx": 0,
>       "ebx": 0
>     },
>     {
>       "eax": 13,
>       "edx": 1231384169,
>       "in-eax": 0,
>       "ecx": 1818588270,
>       "ebx": 1970169159
>     },
>     {
>       "eax": 198354,
>       "edx": 126614527,
>       "in-eax": 1,
>       "ecx": 2176328193,
>       "ebx": 2048
>     },
>     ....
>     {
>       "eax": 12328,
>       "edx": 0,
>       "in-eax": 2147483656,
>       "ecx": 0,
>       "ebx": 0
>     }
>   ]
> }
>
> Signed-off-by: Valeriy Vdovin <valeriy.vdovin@virtuozzo.com>
> ---

[...]

> diff --git a/qapi/machine-target.json b/qapi/machine-target.json
> index e7811654b7..db906c9240 100644
> --- a/qapi/machine-target.json
> +++ b/qapi/machine-target.json
> @@ -329,3 +329,47 @@
>  ##
>  { 'command': 'query-cpu-definitions', 'returns': ['CpuDefinitionInfo'],
>    'if': 'defined(TARGET_PPC) || defined(TARGET_ARM) || defined(TARGET_I386) || defined(TARGET_S390X) || defined(TARGET_MIPS)' }
> +
> +##
> +# @CpuidEntry:
> +#
> +# A single entry of a CPUID response.
> +#
> +# One entry holds full set of information (leaf) returned to the guest
> +# in response to it calling a CPUID instruction with eax, ecx used as
> +# the agruments to that instruction. ecx is an optional argument as

Typo: arguments

> +# not all of the leaves support it.
> +#
> +# @in-eax: CPUID argument in eax
> +# @in-ecx: CPUID argument in ecx
> +# @eax: CPUID result in eax
> +# @ebx: CPUID result in ebx
> +# @ecx: CPUID result in ecx
> +# @edx: CPUID result in edx
> +#
> +# Since: 6.1
> +##
> +{ 'struct': 'CpuidEntry',
> +  'data': { 'in-eax' : 'uint32',
> +            '*in-ecx' : 'uint32',
> +            'eax' : 'uint32',
> +            'ebx' : 'uint32',
> +            'ecx' : 'uint32',
> +            'edx' : 'uint32'
> +          },
> +  'if': 'defined(TARGET_I386)' }
> +
> +##
> +# @query-x86-cpuid:
> +#
> +# Returns raw data from the emulated CPUID table for the first VCPU.
> +# The emulated CPUID table defines the response to the CPUID
> +# instruction when executed by the guest operating system.
> +#
> +# Returns: a list of CpuidEntry
> +#
> +# Since: 6.1
> +##
> +{ 'command': 'query-x86-cpuid',
> +  'returns': ['CpuidEntry'],
> +  'if': 'defined(TARGET_I386)' }

I understand this fails when the acceleator isn't KVM.  I think that's
worth documenting.

Is this intended to be a stable interface?  Interfaces intended just for
debugging usually aren't.

Eduardo, what's your take on this version?