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

[Richard Henderson]

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?


r~