Re: [Qemu-devel] [PATCH v4 0/2] Support CPUID signature for TCG

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, May 09, 2017 at 07:05:51AM -0700, Richard Henderson wrote:
> On 05/09/2017 06:27 AM, Daniel P. Berrange wrote:
> >This enables report of a signature in CPUID for the TCG
> >interpretor.
> >
> >Changed in v4:
> >
> >  - Report 0x40000001 in EAX for 0x40000000 index (Eduardo)
> >  - Report all zeros for 0x40000001 index (Eduardo)
> >  - Make code style consistent when checking limits (Eduardo)
> >
> >Changed in v3:
> >
> >  - Simplify CPU limit code still further (Eduardo)
> >
> >Changed in v2:
> >
> >  - Rewrite the way we bounds check / cap the CPUID index
> >    to use a flat switch, instead of nested ifs (Eduardo)
> >  - Add a 'tcg-cpuid' property to allow it to be hidden
> >    (Eduardo)
> >  - Hide the TCG signature for old machine types
> >  - Force code to a no-op if tcg_enabled() is false (Eduardo)
> >
> >
> >NB, I did not introduce a general 'hypervisor-cpuid' property
> >to obsolete the existing 'kvm=off|on' -cpu property, since it
> >appears impossible to get the back compat semantics right,
> >as described in a previous reply.
> >
> >Daniel P. Berrange (2):
> >   i386: rewrite way CPUID index is validated
> >   i386: expose "TCGTCGTCGTCG" in the 0x40000000 CPUID leaf
> 
> I probably should have commented earlier but...  what's the point?
> 
> If you want the guest os to actually do anything with this, what do
> you gain for advertising TCG over KVM?

I can see this being useful from virt-what, since it would allow
vendors to diagnose problems being caused by TCG (since as Dan
mentions in the other reply and as you are already well acquainted
with, TCG and KVM don't present or emulate quite the same thing).

That said, I don't think it's anything more than a nice to have.

Rich.

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