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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 07/23/2014 02:13 PM, Stefano Stabellini wrote:
> 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.
Right -- sounds good then.

 -George

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel