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

[Eduardo Habkost]

On Tue, May 09, 2017 at 03:13:37PM +0100, Richard W.M. Jones wrote:
> 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.

We also have systemd-detect-virt, and the ConditionVirtualization
option in systemd.

-- 
Eduardo