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

[Avi Kivity <avi@xxxxxxxxxx>]

On 01/16/2012 06:00 AM, Alexander Graf wrote:
> On 16.01.2012, at 04:51, Srivatsa Vaddagiri wrote:
>
> > * 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).
>
> Yes, sorry for not being more exact in my wording. It is a directed yield(). 
> Not like the normal old style thing that just says "I'm done, get some work 
> to someone else" but more something like "I'm done, get some work to this 
> specific guy over there" :).
>

It's not a yield.  It unhalts a vcpu.  Kind of like an IPI, but without
actually issuing an interrupt on the target, and disregarding the
interrupt flag.  It says nothing about the source.

-- 
error compiling committee.c: too many arguments to function


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