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

[Valeriy Vdovin <valeriy.vdovin@virtuozzo.com>]

On Thu, Jun 17, 2021 at 07:22:36AM +0200, Markus Armbruster wrote:
> 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.
> 
Yes, at some point I was thinking this interface could become generic,
but does not seem possible, so I'll remove this note.

> 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?
> 
This CONFIG_KVM is used as a solution to a broken build if --disable-kvm
flag is set. I was choosing between this and writing empty implementation into
kvm-stub.c
> > 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.
> 
It is intented to be used as a stable interface.
> [...]
>