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Subject: Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a
 callee-save function
To: hpa@zytor.com, Peter Zijlstra <peterz@infradead.org>
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Cc: Jeremy Fitzhardinge <jeremy@goop.org>,
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From: Waiman Long <longman@redhat.com>
Organization: Red Hat
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Date: Mon, 13 Feb 2017 17:24:36 -0500
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On 02/13/2017 03:06 PM, hpa@zytor.com wrote:
> On February 13, 2017 2:53:43 AM PST, Peter Zijlstra <peterz@infradead.org> 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.

Cheers,
Longman