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

On February 13, 2017 2:34:01 PM PST, Waiman Long <longman@xxxxxxxxxx> wrote:
>On 02/13/2017 04:52 PM, Peter Zijlstra wrote:
>> On Mon, Feb 13, 2017 at 03:12:45PM -0500, Waiman Long wrote:
>>> On 02/13/2017 02:42 PM, Waiman Long wrote:
>>>> On 02/13/2017 05:53 AM, Peter Zijlstra 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
>>>>>> 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;
>>>>> ?
>>>> Yes, that looks good to me.
>>>> Cheers,
>>>> Longman
>>> Sorry, I am going to take it back. The displacement or offset can
>>> be up to 32-bit. So we will still need to use at least one more
>>> register, I think.
>> I don't think that would be a problem, I very much doubt we declare
>> than 4G worth of per-cpu variables in the kernel.
>> In any case, use "e" or "Z" as constraint (I never quite know when to
>> use which). That are s32 and u32 displacement immediates resp. and
>> should fail compile with a semi-sensible failure if the displacement
>> too big.
>It is the address of &steal_time that will exceed the 32-bit limit.

That seems odd in the extreme?
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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