Re: [Qemu-devel] [RFC 0/2] GET_EMULATED_CPUID support with "allow-emulation" option

[Eduardo Habkost]

On Thu, Jun 05, 2014 at 06:58:17PM +0200, Alexander Graf wrote:
> 
> On 05.06.14 18:52, Paolo Bonzini wrote:
> >Il 05/06/2014 18:45, Alexander Graf ha scritto:
> >>>>
> >>>
> >>>Only if you were using "-cpu somethingThatHasAVX", though, no?
> >>
> >>Yes. The same argument goes the other way around. I want to use AVX
> >>emulation, do "allow-emulation" and suddenly I get MONITOR/MWAIT
> >>emulation.
> >
> >What about:
> >
> >- letting "-cpu foo,+emulatedfeature" just work
> >
> >- adding emulated=yes that blindly enables all emulated features
> >
> >- making "-cpu ...,check" prints a warning for emulated features
> >unless emulated=yes
> 
> So:
> 
>   -cpu foo,+emulatedFeature just works

The problem with this is:

 * -cpu foo,+emulatedFeature,enforce MUST fail. We don't want to enable
   emulated/experimental features by accident, even if they appear in
   the command-line expliclty.
 * -cpu foo,+emulatedFeature (no "enforce" flag) MUST remove remove the
   feature and report it on the "filtered-features" property, because
   that's the only API available for management to check if a feature is
   not available on GET_SUPPORTED_CPUID.

That's why I suggest that the only way to enable emulatedFeature be
using "allow-emulation=yes" explicitly on the command-line IN ADDITION
to already having the feature included in the CPU model definition or in
explicitly in the command-line.

(or "allow-experimental-features", or whatever name we choose)

> 
>   -cpu foo,+notEmulatedFeature still sets the CPUID bit for that feature

That's OK, but (I assume) you mean notEmulatedFeature is on
GET_SUPPORTED_CPUID.

Detailing the requirements:

 * "-cpu foo,+SomeFeature" MUST NOT enable SomeFeature unless it is
   present on GET_SUPPORTED_CPUID.
 * "-cpu foo,+SomeFeature,enforce" MUST abort if the feature is not
   present on GET_SUPPORTED_CPUID.
 * "-cpu foo,+SomeFeature" MUST report SomeFeature on
   "filtered-features", so management knows the feature is not set on
   GET_SUPPORTED_CPUID.

The items above are already part of the semantics of "-cpu" and if we
change it, we risk defeating the whole purpose of GET_EMULATED_CPUID in
the first place.

> 
>   -cpu foo,check prints warnings for all cpuid bits not in the
> "allowed" bitmap. It prints different warnings depending on whether
> the bit is in "emulated" or not

That part makes sense. And we may also report GET_EMULATED_CPUID
features on an "emulated-features" property, so management can get that
information in a machine-friendly way.

-- 
Eduardo