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SubjectRe: [PATCH] ARM: makefile: pass -march=armv4 to assembler even on CPU32v3
(adding Eric since he wrote the ChaCha20 scalar code)

On 2 October 2018 at 09:51, Arnd Bergmann <arnd@arndb.de> wrote:
> On Tue, Oct 2, 2018 at 5:53 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>>
>> Hi Arnd,
>>
>> Apologies for the delay in getting back to you. I had some MTA issues
>> and stupidly assumed ARM developers were taking the day off instead...
>>
>> On Tue, Oct 2, 2018 at 5:33 AM Arnd Bergmann <arnd@arndb.de> wrote:
>> > -arch-$(CONFIG_CPU_32v3) =-D__LINUX_ARM_ARCH__=3 -march=armv3
>> > +arch-$(CONFIG_CPU_32v3) =-D__LINUX_ARM_ARCH__=3 -march=armv3m
>>
>> Unfortunately this doesn't really cut it in my case, as it's not only
>> those multiplications:
>> chacha20-arm.S:402: Error: selected processor does not support `bxeq
>> lr' in ARM mode
>>
>> I think we're going to wind up playing whack-a-mole in silly ways. The
>> fact of the matter is that the ARM assembly I'm adding to the tree is
>> for ARMv4 and up, and not for ARMv3.
>
> I don't see what issues remain. The 'reteq lr' that Ard mentioned
> is definitely the correct way to return from assembly (you also need
> that for plain armv4, as 'bx' was added in armv4t), and Russell
> confirmed that using -march=armv3m is something we want
> anyway for mach-rpc.
>

In fact, this bxeq instruction is the only remaining impediment to
building all scalar code with -march-arm3m, and looking at the code

ENTRY(chacha20_arm)
cmp r2, #0 // len == 0?
bxeq lr

it seems to me that we can move this len == 0 check into the caller instead.

index 163815f51aac..b2108e00d451 100644
--- a/lib/zinc/chacha20/chacha20-arm-glue.h
+++ b/lib/zinc/chacha20/chacha20-arm-glue.h
@@ -59,6 +59,8 @@ static inline bool chacha20_arch(struct chacha20_ctx
*ctx, u8 *dst,
src += bytes;
simd_relax(simd_context);
} else {
+ if (unlikely(!len))
+ break;
chacha20_arm(dst, src, len, ctx->key, ctx->counter);
ctx->counter[0] += (len + 63) / 64;
break;
diff --git a/lib/zinc/chacha20/chacha20-arm.S b/lib/zinc/chacha20/chacha20-arm.S
index 5abedafcf129..845843a14ab1 100644
--- a/lib/zinc/chacha20/chacha20-arm.S
+++ b/lib/zinc/chacha20/chacha20-arm.S
@@ -398,9 +398,6 @@
* const u32 iv[4]);
*/
ENTRY(chacha20_arm)
- cmp r2, #0 // len == 0?
- bxeq lr
-
push {r0-r2,r4-r11,lr}

// Push state x0-x15 onto stack.
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