Re: [PATCH v2 5/6] kvm: x86: Emulate MSR IA32_CORE_CAPABILITIES

[Andy Lutomirski]

On Tue, Feb 4, 2020 at 1:37 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Feb 04, 2020 at 05:19:26PM +0800, Xiaoyao Li wrote:
>
> > > > + case MSR_IA32_CORE_CAPS:
> > > > +         if (!msr_info->host_initiated)
> > >
> > > Shouldn't @data be checked against kvm_get_core_capabilities()?
> >
> > Maybe it's for the case that userspace might have the ability to emulate SLD
> > feature? And we usually let userspace set whatever it wants, e.g.,
> > ARCH_CAPABILITIES.
>
> If the 'sq_misc.split_lock' event is sufficiently accurate, I suppose
> the host could use that to emulate the feature at the cost of one
> counter used.

I would be impressed if the event were to fire before executing the
offending split lock.  Wouldn't the best possible result be for it to
fire with RIP pointing to the *next* instruction?  This seems like it
could be quite confusing to a guest.