Re: [PATCH v8] qapi: introduce 'query-kvm-cpuid' action

[Philippe Mathieu-Daudé]

Hi Valeriy,

(Sorry for not looking earlier than v8)

On 5/31/21 2:38 PM, Valeriy Vdovin wrote:
> Introducing new qapi method 'query-kvm-cpuid'. This method 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.
> 
> Currently full output for this method is only supported for x86 cpus.
> 
> 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'
> 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
> return 4 words in registers EAX, EBX, ECX, EDX.
> 
> Use example:
> qmp_request: {
>   "execute": "query-kvm-cpuid"
> }
> 
> qmp_response: [
>   {
>     "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,
>   ....
> 
> 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
> ---

^ Below this '---' go your comments.

>  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..a83180dd24 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
> +# instruction. ecx is an optional argument as not all of the leaves support 
> it.
> +#
> +# @in_eax: CPUID argument in eax
> +# @in_ecx: CPUID argument in ecx
> +# @eax: eax
> +# @ebx: ebx
> +# @ecx: ecx
> +# @edx: 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) && defined(CONFIG_KVM)' }

What about other accelerators?

> +
> +##
> +# @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.

What is specific to KVM here?

What about 'query-accel-cpuid' or 'query-vm-cpu-id'?

> +#
> +# Returns: a list of CpuidEntry
> +#
> +# Since: 6.1
> +##
> +{ 'command': 'query-kvm-cpuid',
> +  'returns': ['CpuidEntry'],
> +  'if': 'defined(TARGET_I386) && defined(CONFIG_KVM)' }
...
> diff --git a/tests/qtest/qmp-cmd-test.c b/tests/qtest/qmp-cmd-test.c
> index c98b78d033..48add3ada1 100644
> --- a/tests/qtest/qmp-cmd-test.c
> +++ b/tests/qtest/qmp-cmd-test.c
> @@ -46,6 +46,7 @@ static int query_error_class(const char *cmd)
>          { "query-balloon", ERROR_CLASS_DEVICE_NOT_ACTIVE },
>          { "query-hotpluggable-cpus", ERROR_CLASS_GENERIC_ERROR },
>          { "query-vm-generation-id", ERROR_CLASS_GENERIC_ERROR },
> +        { "query-kvm-cpuid", ERROR_CLASS_GENERIC_ERROR },
>          { NULL, -1 }
>      };
>      int i;
>