Re: [Xen-devel] [PATCH RFC V4 5/5] Documentation/kvm : Add documentation on Hypercalls and features used for PV spinlock

[Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>]

* Alexander Graf <agraf@xxxxxxx> [2012-01-16 04:23:24]:

> > +5. KVM_HC_KICK_CPU
> > +------------------------
> > +value: 5
> > +Architecture: x86
> > +Purpose: Hypercall used to wakeup a vcpu from HLT state
> > +
> > +Usage example : A vcpu of a paravirtualized guest that is busywaiting in 
> > guest
> > +kernel mode for an event to occur (ex: a spinlock to become available)
> > +can execute HLT instruction once it has busy-waited for more than a
> > +threshold time-interval. Execution of HLT instruction would cause
> > +the hypervisor to put the vcpu to sleep (unless yield_on_hlt=0) until 
> > occurence
> > +of an appropriate event. Another vcpu of the same guest can wakeup the 
> > sleeping
> > +vcpu by issuing KVM_HC_KICK_CPU hypercall, specifying APIC ID of the vcpu 
> > to be
> > +wokenup.
> 
> The description is way too specific. The hypercall basically gives the guest 
> the ability to yield() its current vcpu to another chosen vcpu.

Hmm ..the hypercall does not allow a vcpu to yield. It just allows some
target vcpu to be prodded/wokenup, after which vcpu continues execution.

Note that semantics of this hypercall is different from the hypercall on which
PPC pv-spinlock (__spin_yield()) is currently dependent. This is mainly because 
of ticketlocks on x86 (which does not allow us to easily store owning cpu
details in lock word itself).

> The APIC piece is an implementation detail for x86. On PPC we could just use 
> the PIR register contents (processor identifier).

- vatsa


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