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

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jun 02, 2021 at 08:17:28PM +0200, Philippe Mathieu-Daudé wrote:
> 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'?

The implementation is KVM-specific.  I believe it's a reasonable
compromise because the implementation is trivial, and a raw copy
of the KVM CPUID table makes it a more useful (KVM-specific)
debugging/testing mechanism.

I don't really mind how the command is called, but I would prefer
to add a more complex abstraction only if maintainers of other
accelerators are interested and volunteer to provide similar
functionality.  I don't want to introduce complexity for use
cases that may not even exist.

> 
> > +#
> > +# 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;
> > 
> 

-- 
Eduardo