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

[George Dunlap <george.dunlap@eu.citrix.com>]

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@eu.citrix.com> wrote:
>>> No need to call vcpu_force_reschedule, is too expensive.
>>>
>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
>>> ---
>>>   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