Re: [PATCH v16] qapi: introduce 'x-query-x86-cpuid' QMP command.

Markus Armbruster
2022-01-19T14:38:07Z

Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes:

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

Please wrap your commit messages around column 70.

>
> 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": "x-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>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

This needs review from x86 CPU maintainers. Eduardo?

[...]

> diff --git a/qapi/machine-target.json b/qapi/machine-target.json
> index f5ec4bc172..0ac575b1b9 100644
> --- a/qapi/machine-target.json
> +++ b/qapi/machine-target.json
> @@ -341,3 +341,49 @@
> 'TARGET_I386',
> 'TARGET_S390X',
> '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 arguments 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: CPUID result in eax
> +# @ebx: CPUID result in ebx
> +# @ecx: CPUID result in ecx
> +# @edx: CPUID result in edx
> +#
> +# Since: 7.0
> +##
> +{ 'struct': 'CpuidEntry',
> + 'data': { 'in-eax' : 'uint32',
> + '*in-ecx' : 'uint32',
> + 'eax' : 'uint32',
> + 'ebx' : 'uint32',
> + 'ecx' : 'uint32',
> + 'edx' : 'uint32'
> + },
> + 'if': 'TARGET_I386' }
> +
> +##
> +# @x-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. Returns error when qemu is configured with
> +# --disable-kvm flag or if qemu is run with any other accelerator than KVM.

Long line, please wrap around column 70.

> +#
> +# Since: 7.0
> +##
> +{ 'command': 'x-query-x86-cpuid',
> + 'returns': ['CpuidEntry'],
> + 'if': 'TARGET_I386' }

Needs feature 'unstable' now; see commit a3c45b3e62 'qapi: New special
feature flag "unstable"' and also commit 57df0dff1a "qapi: Extend
-compat to set policy for unstable interfaces". Incremental patch
appended for your convenience.

diff --git a/qapi/machine-target.json b/qapi/machine-target.json
index 0ac575b1b9..049fa48a35 100644
--- a/qapi/machine-target.json
+++ b/qapi/machine-target.json
@@ -378,6 +378,8 @@
# The emulated CPUID table defines the response to the CPUID
# instruction when executed by the guest operating system.
#
+# Features:
+# @unstable: This command is experimental.
#
# Returns: a list of CpuidEntry. Returns error when qemu is configured with
# --disable-kvm flag or if qemu is run with any other accelerator than KVM.
@@ -386,4 +388,5 @@
##
{ 'command': 'x-query-x86-cpuid',
'returns': ['CpuidEntry'],
- 'if': 'TARGET_I386' }
+ 'if': 'TARGET_I386',
+ 'features': [ 'unstable' ] }