Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Apr 20, 2012 at 12:12:38PM +0200, Jan Kiszka wrote:
> On 2012-04-19 22:03, Eduardo Habkost wrote:
> > Jan/Avi: ping?
> > 
> > I would like to get this ABI detail clarified so it can be implemented
> > the right way on Qemu and KVM.
> > 
> > My proposal is to simply add tsc-deadline to the data returned by
> > GET_SUPPORTED_CPUID, making KVM_CAP_TSC_DEADLINE_TIMER unnecessary.
> > 
> 
> IIRC, there were problems with this model to exclude that the feature
> gets reported this way without ensuring that in-kernel irqchip is
> actually activated. Please browse the discussions, it should be
> documented there.

The flag was never added to GET_SUPPORTED_CPUID on any of the git
repositories I have checked, and I don't see that being done anywhere on
my KVM mailing list archives, either. So I don't see how we could have
had issues with GET_SUPPORTED_CPUID, if it was never present on the
code.

What was present on the code before the fix, was coded that
unconditinally enabled the flag even if Qemu never asked for it, and
that really was wrong.

GET_SUPPORTED_CPUID doesn't enable any flag: it just tells userspace
that the hardware and KVM supports the feature (and, surprise, this is
exactly what KVM_CAP_TSC_DEADLINE_TIMER means too, isn't it?). It's up
to userspace to enable the CPUID bits according to user requirements and
userspace capabilities (in other words: only when userspace knows it is
safe because the in-kernel irqchip is enabled).

If the above is not what GET_SUPPORTED_CPUID means, I would like to get
that clarified, so I can submit an updated to KVM API documentation.

-- 
Eduardo