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

[hpa@zytor.com]

On February 13, 2017 1:52:20 PM PST, Peter Zijlstra <peterz@infradead.org> 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
>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;
>> >>
>> >> ?
>> > Yes, that looks good to me.
>> >
>> > Cheers,
>> > Longman
>> >
>> Sorry, I am going to take it back. The displacement or offset can
>only
>> 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
>more
>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 is
>too big.

e for signed, Z for unsigned.  Obviously you have to use a matching instruction: an immediate or displacement in a 64-bit instruction is sign-extended, in a 32-bit instruction zero-extended.  E.g.:

   movl %0,%%eax # use Z, all of %rax will be set
   movq %0,%%rax # use e
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