On Wed, Dec 06, 2017 at 12:25:44PM +0000, Ard Biesheuvel wrote:

On 6 December 2017 at 12:12, Dave P Martin [off-list ref] wrote:

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On Wed, Dec 06, 2017 at 11:57:12AM +0000, Ard Biesheuvel wrote:

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On 6 December 2017 at 11:51, Dave Martin [off-list ref] wrote:

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On Tue, Dec 05, 2017 at 06:04:34PM +0000, Ard Biesheuvel wrote:

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On 5 December 2017 at 12:45, Ard Biesheuvel [off-list ref] wrote:

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On 5 Dec 2017, at 12:28, Dave Martin [off-list ref] wrote:

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If we had

   if_cond_yield_neon
   // patchup code
   endif_yield_neon

I like this, but ...

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then the caller is free to conditionally branch over that as appropriate
like

loop:
   // crypto stuff
   tst x0, #0xf
   b.ne    loop

   if_cond_yield_neon
   // patchup code
   endif_cond_yield_neon

I need to put the post patchup code somewhere too. Please refer to the
CRC patches for the best examples of this.

I'm not sure I follow.  If you need to do something different after
patchup, can't you either pull that code into the if...endif too, or
otherwise put a branch before the endif?

No, not really.

What I currently have is

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   if_cond_yield_neon
   // patchup code
   endif_cond_yield_neon

which is being turned into

    get_thread_info x0
    ldr w1, [x0, #TSK_TI_PREEMPT]
    ldr x0, [x0, #TSK_TI_FLAGS]
    cmp w1, #1 // == PREEMPT_OFFSET
    csel x0, x0, xzr, eq
    tbnz x0, #TIF_NEED_RESCHED, 5555f // needs rescheduling?
4444:

    .subsection 1
5555:
    // patchup code
    bl kernel_neon_end
    bl kernel_neon_begin
    // post patchup code goes here
    b 4444b
    .subsection

so what I basically need is a place to put the post patchup code,
unless I open code it and branch to a different label right after
kernel_neon_begin

Ah, I get you.  I had convinced myself that there was only post-
patchup code.

So you conceptually want something like

if_will_cond_yield_neon
        // pre-yield patchup code
do_cond_yield_neon
        // post-yield patchup code
endif_yield_neon

.macro if_will_cond_yield_neon
        // do preempt_count == 1 && TIF_NEED_RESCHED check
        b.wont_yield 3432f
.endm

.macro do_cond_yield_neon
        bl      kernel_neon_end
        bl      kernel_neon_begin
.endm

.macro endif_yield_neon
3432:
.endm

The main difference I see is that this doesn't get the slow-case
code out of line.

Or have I still missed something?

No, that seems accurate. I think the idiom looks much better, but I'd
still like to get the code out of line, so that everything disappears
completely for !CONFIG_PREEMPT.

Sure, out-of-lining should still be possible by spilling into another
section or subsection similarly to what you were already doing -- and
it may still make sense to have (optional?) label arguments to allow
some branch elision.  I didn't want to complicate things initially.

I'll try to rework my stuff using this as a starting point.

Cool, let me know.

Cheers
---Dave