Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function

From: Peter Zijlstra
Date: Mon Feb 13 2017 - 05:48:47 EST

On Fri, Feb 10, 2017 at 12:00:43PM -0500, Waiman Long wrote:

> >> +asm(
> >> +".pushsection .text;"
> >> +".global __raw_callee_save___kvm_vcpu_is_preempted;"
> >> +".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
> >> +"__raw_callee_save___kvm_vcpu_is_preempted:"
> >> +"push %rdi;"
> >> +"push %rdx;"
> >> +"movslq %edi, %rdi;"
> >> +"movq $steal_time+16, %rax;"
> >> +"movq __per_cpu_offset(,%rdi,8), %rdx;"
> >> +"cmpb $0, (%rdx,%rax);"

Could we not put the $steal_time+16 displacement as an immediate in the
cmpb and save a whole register here?

That way we'd end up with something like:

push %rdi;
movslq %edi, %rdi;
movq __per_cpu_offset(,%rdi,8), %rax;
cmpb $0, %[offset](%rax);
setne %al;
pop %rdi;
" : : [offset] "i" (((unsigned long)&steal_time) + offsetof(struct steal_time, preempted)));

And if we could get rid of the sign extend on edi we could avoid all the
push-pop nonsense, but I'm not sure I see how to do that (then again,
this asm foo isn't my strongest point).

> >> +"setne %al;"
> >> +"pop %rdx;"
> >> +"pop %rdi;"
> >> +"ret;"
> >> +".popsection");
> >> +
> >> +#endif
> >> +
> >> /*
> >> * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
> >> */
> > That should work for now. I have done something similar for
> > __pv_queued_spin_unlock. However, this has the problem of creating a
> > dependency on the exact layout of the steal_time structure. Maybe the
> > constant 16 can be passed in as a parameter offsetof(struct
> > kvm_steal_time, preempted) to the asm call.

Yeah it should be well possible to pass that in. But ideally we'd have
GCC grow something like __attribute__((callee_saved)) or somesuch and it
would do all this for us.

> One more thing, that will improve KVM performance, but it won't help Xen.

People still use Xen? ;-) In any case, their implementation looks very
similar and could easily crib this.

> I looked into the assembly code for rwsem_spin_on_owner, It need to save
> and restore 2 additional registers with my patch. Doing it your way,
> will transfer the save and restore overhead to the assembly code.
> However, __kvm_vcpu_is_preempted() is called multiple times per
> invocation of rwsem_spin_on_owner. That function is simple enough that
> making __kvm_vcpu_is_preempted() callee-save won't produce much compiler
> optimization opportunity.

This is because of that noinline, right? Otherwise it would've been
folded and register pressure would be much higher.

> The outer function rwsem_down_write_failed()
> does appear to be a bit bigger (from 866 bytes to 884 bytes) though.

I suspect GCC is being clever and since all this is static it plays
games with the calling convention and pushes these clobbers out.