Re: [PATCH v3 11/20] arm64: assembler: add macros to conditionally yield the NEON under PREEMPT

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 7 December 2017 at 14:39, Dave Martin <Dave.Martin@arm.com> wrote:
> On Wed, Dec 06, 2017 at 07:43:37PM +0000, Ard Biesheuvel wrote:
>> Add support macros to conditionally yield the NEON (and thus the CPU)
>> that may be called from the assembler code.
>>
>> 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 three, and the code in between is only
>> executed when the yield path is taken, allowing the context to be preserved.
>> The third macro takes an optional label argument that marks the resume
>> path after a yield has been performed.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>>  arch/arm64/include/asm/assembler.h | 51 ++++++++++++++++++++
>>  1 file changed, 51 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
>> index 5f61487e9f93..c54e408fd5a7 100644
>> --- a/arch/arm64/include/asm/assembler.h
>> +++ b/arch/arm64/include/asm/assembler.h
>> @@ -572,4 +572,55 @@ alternative_else_nop_endif
>>  #endif
>>       .endm
>>
>> +/*
>> + * Check whether to yield to another runnable task from kernel mode NEON code
>> + * (which runs with preemption disabled).
>> + *
>> + * if_will_cond_yield_neon
>> + *        // pre-yield patchup code
>> + * do_cond_yield_neon
>> + *        // post-yield patchup code
>> + * endif_yield_neon
>
> ^ Mention the lbl argument?
>

Yep will do

>> + *
>> + * - Check whether the preempt count is exactly 1, in which case disabling
>
>                                                            enabling ^
>
>> + *   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.
>
> Mention that neither patchup sequence is allowed to use section-changing
> directives?
>
> For example:
>
>         if_will_cond_yield_neon
>                 // some code
>
>                 .pushsection .rodata, "a"
> foo:                    .quad // some literal data for some reason
>                 .popsection
>
>                 // some code
>         do_cond_yield_neon
>
> is not safe, because .previous is now .rodata.
>

Are you sure this is true?

The gas info page for .previous tells me

   In terms of the section stack, this directive swaps the current
section with the top section on the section stack.

and it seems to me that .rodata is no longer on the section stack
after .popsection. In that sense, push/pop should be safe, but
section/subsection/previous is not (I think). So yes, let's put a note
in to mention that section directives are unsupported.

> (You could protect against this with
>         .pushsection .text; .previous; .subsection 1; // ...
>         .popsection
> but it may be overkill.)
>
>> + *
>> + * This macro sequence clobbers x0, x1 and the flags register unconditionally,
>> + * and may clobber x2 .. x18 if the yield path is taken.
>> + */
>> +
>> +     .macro          cond_yield_neon, lbl
>> +     if_will_cond_yield_neon
>> +     do_cond_yield_neon
>> +     endif_yield_neon        \lbl
>> +     .endm
>> +
>> +     .macro          if_will_cond_yield_neon
>> +#ifdef CONFIG_PREEMPT
>> +     get_thread_info x0
>> +     ldr             w1, [x0, #TSK_TI_PREEMPT]
>> +     ldr             x0, [x0, #TSK_TI_FLAGS]
>> +     cmp             w1, #1 // == PREEMPT_OFFSET
>
> Can we at least drop a BUILD_BUG_ON() somewhere to check this?
>
> Maybe in kernel_neon_begin() since this is intimately kernel-mode NEON
> related.
>

Sure.

>> +     csel            x0, x0, xzr, eq
>> +     tbnz            x0, #TIF_NEED_RESCHED, 5555f    // needs rescheduling?
>> +#endif
>
> A comment that we will fall through to 6666f here may be helpful.
>

Indeed. Will add that.

>> +     .subsection     1
>> +5555:
>> +     .endm
>> +
>> +     .macro          do_cond_yield_neon
>> +     bl              kernel_neon_end
>> +     bl              kernel_neon_begin
>> +     .endm
>> +
>> +     .macro          endif_yield_neon, lbl=6666f
>> +     b               \lbl
>> +     .previous
>> +6666:
>
> Could have slightly more random "random" labels here, but otherwise
> it looks ok to me.
>

Which number did you have in mind that is more random than 6666? :-)

> I might go through and replace all the random labels with something
> more robust sometime, but I've never been sure it was worth the
> effort...
>

I guess we could invent all kinds of elaborate schemes but as you say,
having 4 digit numbers and grep'ing the source before you add a new
one has been working fine so far, so I don't think it should be a
priority.