On Wed, Sep 26, 2018 at 5:45 PM Jason A. Donenfeld [off-list ref] wrote:

So what you have in mind is something like calling simd_relax() every
4096 bytes or so?

That was actually pretty easy, putting together both of your suggestions:

static inline bool chacha20_arch(struct chacha20_ctx *state, u8 *dst,
                 u8 *src, size_t len,
                 simd_context_t *simd_context)
{
    while (len > PAGE_SIZE) {
        chacha20_arch(state, dst, src, PAGE_SIZE, simd_context);
        len -= PAGE_SIZE;
        src += PAGE_SIZE;
        dst += PAGE_SIZE;
        simd_relax(simd_context);
    }
    if (IS_ENABLED(CONFIG_KERNEL_MODE_NEON) && chacha20_use_neon &&
        len >= CHACHA20_BLOCK_SIZE * 3 && simd_use(simd_context))
        chacha20_neon(dst, src, len, state->key, state->counter);
    else
        chacha20_arm(dst, src, len, state->key, state->counter);

    state->counter[0] += (len + 63) / 64;
    return true;
}