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

From: Pan Xinhui
Date: Thu Nov 17 2016 - 00:16:57 EST

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.


Also, it generates more code.

Paravirt muck should strive to be as cheap as possible when ran on
native hardware.