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

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

quoted

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

quoted

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

quoted

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

quoted

quoted

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

quoted

On Mon, Dec 04, 2017 at 12:26:37PM +0000, Ard Biesheuvel wrote:
Add a support macro to conditionally yield the NEON (and thus the CPU)
that may be called from the assembler code. Given that especially the
instruction based accelerated crypto code may use very tight loops, add
some parametrization so that the TIF_NEED_RESCHED flag test is only
executed every so many loop iterations.

In some cases, yielding the NEON involves saving and restoring a non
trivial amount of context (especially in the CRC folding algorithms),
and so the macro is split into two, and the code in between is only
executed when the yield path is taken, allowing the contex to be preserved.
The second macro takes a label argument that marks the resume-from-yield
path, which should restore the preserved context again.

Signed-off-by: Ard Biesheuvel <redacted>
---
arch/arm64/include/asm/assembler.h | 50 ++++++++++++++++++++
1 file changed, 50 insertions(+)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index aef72d886677..917b026d3e00 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h

@@ -512,4 +512,54 @@ alternative_else_nop_endif

#endif
   .endm

+/*
+ * yield_neon - check whether to yield to another runnable task from
+ *        kernel mode NEON code (running with preemption disabled)
+ *
+ * - Check whether the preempt count is exactly 1, in which case disabling
+ *   preemption once will make the task preemptible. If this is not the case,
+ *   yielding is pointless.
+ * - Check whether TIF_NEED_RESCHED is set, and if so, disable and re-enable
+ *   kernel mode NEON (which will trigger a reschedule), and branch to the
+ *   yield fixup code at @lbl.
+ */
+    .macro        yield_neon, lbl:req, ctr, order, stride, loop
+    yield_neon_pre    \ctr, \order, \stride, \loop
+    yield_neon_post    \lbl
+    .endm
+
+    .macro        yield_neon_pre, ctr, order=0, stride, loop=4444f
+#ifdef CONFIG_PREEMPT
+    /*
+     * With some algorithms, it makes little sense to poll the
+     * TIF_NEED_RESCHED flag after every iteration, so only perform
+     * the check every 2^order strides.
+     */
+    .if        \order > 1
+    .if        (\stride & (\stride - 1)) != 0
+    .error        "stride should be a power of 2"
+    .endif
+    tst        \ctr, #((1 << \order) * \stride - 1) & ~(\stride - 1)
+    b.ne        \loop
+    .endif

I'm not sure what baking in this check gives us, and this seems to
conflate two rather different things: yielding and defining a
"heartbeat" frequency for the calling code.

Can we separate out the crypto-loop-helper semantics from the yield-
neon stuff?

Fair enough. I incorporated the check here so it disappears from the code entirely when !CONFIG_PREEMPT, because otherwise, you end up with a sequence that is mispredicted every # iterations without any benefit.
I guess i could macroise that separately though.

quoted

If we had

   if_cond_yield_neon
   // patchup code
   endif_yield_neon

I like this, but ...

quoted

quoted

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

quoted

   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.

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