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

From: Waiman Long
Date: Mon Feb 13 2017 - 17:24:43 EST

On 02/13/2017 03:06 PM, hpa@xxxxxxxxx wrote:
> On February 13, 2017 2:53:43 AM PST, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> On Mon, Feb 13, 2017 at 11:47:16AM +0100, Peter Zijlstra wrote:
>>> That way we'd end up with something like:
>>> asm("
>>> 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).
>> Maybe:
>> movsql %edi, %rax;
>> movq __per_cpu_offset(,%rax,8), %rax;
>> cmpb $0, %[offset](%rax);
>> setne %al;
>> ?
> We could kill the zero or sign extend by changing the calling interface to pass an unsigned long instead of an int. It is much more likely that a zero extend is free for the caller than a sign extend.

I have thought of that too. However, the goal is to eliminate memory
read/write from/to stack. Eliminating a register sign-extend instruction
won't help much in term of performance.