# Re: \[PATCH v2\] x86/paravirt: Don't make vcpu\_is\_preempted() a callee-save function

- From: hpa@zytor.com
- Date: 2017-02-13 20:13:09 
- List: virtualization
- Page: https://ratatoskr.run/virtualization/2017/02/4933545
- Thread: https://ratatoskr.run/linux-arch/2017/02/4932016/t.md

```text
On February 13, 2017 2:53:43 AM PST, Peter Zijlstra [ref] 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.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.
```