Re: [PATCH v7 06/11] x86, paravirt: Add interface to support kvm/xen vcpu preempted check

[Pan Xinhui]

å 2016/11/16 18:23, Peter Zijlstra åé:
> On Wed, Nov 16, 2016 at 12:19:09PM +0800, Pan Xinhui wrote:
> > Hi, Peter.
> > 	I think we can avoid a function call in a simpler way. How about below
> >
> > static inline bool vcpu_is_preempted(int cpu)
> > {
> > 	/* only set in pv case*/
> > 	if (pv_lock_ops.vcpu_is_preempted)
> > 		return pv_lock_ops.vcpu_is_preempted(cpu);
> > 	return false;
> > }
>
> That is still more expensive. It needs to do an actual load and makes it
> hard to predict the branch, you'd have to actually wait for the load to
> complete etc.
yes, one more load in native case. I think this is acceptable as vcpu_is_preempted is not a critical function.

however if we use pv_callee_save_regs_thunk, more unnecessary registers might be save/resotred in pv case.
that will introduce a little overhead.

but I think I am okay with your idea. I can make another patch based on this patchset with your suggested-by.

thanks
xinhui

> Also, it generates more code.
>
> Paravirt muck should strive to be as cheap as possible when ran on
> native hardware.