Re: [Xen-devel] [PATCH 2/2] xen/arm: call vcpu_yield on WFE trap

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 23 Jul 2014, George Dunlap wrote:
> On Wed, Jul 23, 2014 at 1:05 PM, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > No need to call vcpu_force_reschedule, is too expensive.
> >
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > ---
> >  xen/arch/arm/traps.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> > index 3dfabd0..8cd06cc 100644
> > --- a/xen/arch/arm/traps.c
> > +++ b/xen/arch/arm/traps.c
> > @@ -1805,7 +1805,7 @@ asmlinkage void do_trap_hypervisor(struct 
> > cpu_user_regs *regs)
> >          }
> >          if ( hsr.wfi_wfe.ti ) {
> >              /* Yield the VCPU for WFE */
> > -            vcpu_force_reschedule(current);
> > +            vcpu_yield(current);
> 
> Actually, sorry -- why are you wanting to yield here?
> 
> At the moment "yield" is only initiated by the guest itself, and the
> individual schedulers are notified that the guest has called "yield".
> If what you want to do is yield for some other reason, it might be
> better to have a slightly separate path for that.
 
It is a very similar situation: WFE means "wait for event" and it is
used in the implementation of spin_locks, for example.
I think that trapping the instruction and yielding is what we want.

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