Re: [PATCH v9] qapi: introduce 'query-kvm-cpuid' action

[Markus Armbruster <armbru@redhat.com>]

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

> Introducing new qapi method 'query-kvm-cpuid'. This method can be used to

It's actually a QMP command.  There are no "qapi methods".

> 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'

virtual 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.
>
> Currently full output for this method is only supported for x86 cpus.

Not sure about "currently": the interface looks quite x86-specific to me.

The commit message doesn't mention KVM except in the command name.  The
schema provides the command only if defined(CONFIG_KVM).

Can you explain why you need the restriction to CONFIG_KVM?

> To learn exactly how virtual cpu is presented to the guest machine via CPUID
> instruction, new qapi method can be used. By calling 'query-kvm-cpuid'

QMP command again.

> method, one can get a full listing of all CPUID leafs with subleafs which are
> supported by the initialized virtual cpu.
>
> Other than debug, the method 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.
>
> Output format:
> The output is a plain list of leaf/subleaf agrument combinations, that

Typo: argument

> return 4 words in registers EAX, EBX, ECX, EDX.
>
> Use example:
> qmp_request: {
>   "execute": "query-kvm-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>
> ---
> v2: - Removed leaf/subleaf iterators.
>     - Modified cpu_x86_cpuid to return false in cases when count is
>       greater than supported subleaves.
> v3: - Fixed structure name coding style.
>     - Added more comments
>     - Ensured buildability for non-x86 targets.
> v4: - Fixed cpu_x86_cpuid return value logic and handling of 0xA leaf.
>     - Fixed comments.
>     - Removed target check in qmp_query_cpu_model_cpuid.
> v5: - Added error handling code in qmp_query_cpu_model_cpuid
> v6: - Fixed error handling code. Added method to query_error_class
> v7: - Changed implementation in favor of cached cpuid_data for
>       KVM_SET_CPUID2
> v8: - Renamed qmp method to query-kvm-cpuid and some fields in response.
>     - Modified documentation to qmp method
>     - Removed helper struct declaration
> v9: - Renamed 'in_eax' / 'in_ecx' fields to 'in-eax' / 'in-ecx'
>     - Pasted more complete response to commit message.
>
>  qapi/machine-target.json   | 43 ++++++++++++++++++++++++++++++++++++++
>  target/i386/kvm/kvm.c      | 37 ++++++++++++++++++++++++++++++++
>  tests/qtest/qmp-cmd-test.c |  1 +
>  3 files changed, 81 insertions(+)
>
> diff --git a/qapi/machine-target.json b/qapi/machine-target.json
> index e7811654b7..1e591ba481 100644
> --- a/qapi/machine-target.json
> +++ b/qapi/machine-target.json
> @@ -329,3 +329,46 @@
>  ##
>  { '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

Typi: agruments

> +# instruction. ecx is an optional argument as not all of the leaves support it.

Please wrap doc comment lines around column 70.

> +#
> +# @in-eax: CPUID argument in eax
> +# @in-ecx: CPUID argument in ecx
> +# @eax: eax
> +# @ebx: ebx
> +# @ecx: ecx
> +# @edx: edx

Suggest

   # @eax: CPUID result in eax

and so forth.

> +#
> +# Since: 6.1
> +##
> +{ 'struct': 'CpuidEntry',
> +  'data': { 'in-eax' : 'uint32',
> +            '*in-ecx' : 'uint32',
> +            'eax' : 'uint32',
> +            'ebx' : 'uint32',
> +            'ecx' : 'uint32',
> +            'edx' : 'uint32'
> +          },
> +  'if': 'defined(TARGET_I386) && defined(CONFIG_KVM)' }
> +
> +##
> +# @query-kvm-cpuid:
> +#
> +# Returns raw data from the KVM CPUID table for the first VCPU.
> +# The KVM 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-kvm-cpuid',
> +  'returns': ['CpuidEntry'],
> +  'if': 'defined(TARGET_I386) && defined(CONFIG_KVM)' }

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

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