Re: [PATCH v14] qapi: introduce 'query-x86-cpuid' QMP command.

From: Eduardo Habkost <ehabkost@redhat.com>
To: Valeriy Vdovin <valery.vdovin.s@gmail.com>
Cc: Laurent Vivier <lvivier@redhat.com>,
	Thomas Huth <thuth@redhat.com>,
	Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	kvm@vger.kernel.org, Marcelo Tosatti <mtosatti@redhat.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	qemu-devel@nongnu.org, Markus Armbruster <armbru@redhat.com>,
	Valeriy Vdovin <valeriy.vdovin@virtuozzo.com>,
	Denis Lunev <den@openvz.org>, Paolo Bonzini <pbonzini@redhat.com>,
	Eric Blake <eblake@redhat.com>
Subject: Re: [PATCH v14] qapi: introduce 'query-x86-cpuid' QMP command.
Date: Tue, 10 Aug 2021 15:50:53 -0400	[thread overview]
Message-ID: <20210810195053.6vsjadglrexf6jwy@habkost.net> (raw)
In-Reply-To: <20210810065131.2849-1-valery.vdovin.s@gmail.com>

On Tue, Aug 10, 2021 at 09:51:31AM +0300, Valeriy Vdovin wrote:
> 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.
> 
> Output format:
> The output is a plain list of leaf/subleaf argument combinations, that
> return 4 words in registers EAX, EBX, ECX, EDX.
>
[...]

Based on the effort being required from you to make sure this
patch is in good shape, maybe you could reconsider my suggestion
from a while ago for a single-CPUID-leaf interface, as discussed
at:
https://lore.kernel.org/qemu-devel/20210421201759.utsmhuopdmlhghbx@habkost.net/

A single-CPUID-leaf qmp_query_x86_cpuid() function that is
generic and not KVM-specific can probably be implemented in ~5
lines of code.

I'm not against the interface proposed here, but you are surely
going to get more friction and more complexity to deal with.

-- 
Eduardo