On February 13, 2017 1:52:20 PM PST, Peter Zijlstra [off-list ref] wrote:

On Mon, Feb 13, 2017 at 03:12:45PM -0500, Waiman Long wrote:

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On 02/13/2017 02:42 PM, Waiman Long wrote:

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On 02/13/2017 05:53 AM, Peter Zijlstra wrote:

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On Mon, Feb 13, 2017 at 11:47:16AM +0100, Peter Zijlstra wrote:

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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)));

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And if we could get rid of the sign extend on edi we could avoid

all the

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push-pop nonsense, but I'm not sure I see how to do that (then

again,

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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

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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.

Oh, and unless you are explicitly forcing 32-bit addressing mode, displacements are always "e" (or "m" if you let gcc pick the addressing mode.)
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