On Wed, Oct 02, 2019 at 04:16:55PM +0200, Ard Biesheuvel wrote:
> Wire the existing x86 SIMD ChaCha code into the new ChaCha library
> interface.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> ---
> arch/x86/crypto/chacha_glue.c | 36 ++++++++++++++++++++
> crypto/Kconfig | 1 +
> include/crypto/chacha.h | 6 ++++
> 3 files changed, 43 insertions(+)
>
> diff --git a/arch/x86/crypto/chacha_glue.c b/arch/x86/crypto/chacha_glue.c
> index bc62daa8dafd..fd9ef42842cf 100644
> --- a/arch/x86/crypto/chacha_glue.c
> +++ b/arch/x86/crypto/chacha_glue.c
> @@ -123,6 +123,42 @@ static void chacha_dosimd(u32 *state, u8 *dst, const u8 *src,
> }
> }
>
> +void hchacha_block(const u32 *state, u32 *stream, int nrounds)
> +{
> + state = PTR_ALIGN(state, CHACHA_STATE_ALIGN);
> +
> + if (!crypto_simd_usable()) {
> + hchacha_block_generic(state, stream, nrounds);
> + } else {
> + kernel_fpu_begin();
> + hchacha_block_ssse3(state, stream, nrounds);
> + kernel_fpu_end();
> + }
> +}
> +EXPORT_SYMBOL(hchacha_block);
Please correct me if I'm wrong:
The approach here is slightly different from Zinc. In Zinc, I had one
entry point that conditionally called into the architecture-specific
implementation, and I did it inline using #includes so that in some
cases it could be optimized out.
Here, you override the original symbol defined by the generic module
from the architecture-specific implementation, and in there you decide
which way to branch.
Your approach has the advantage that you don't need to #include a .c
file like I did, an ugly yet very effective approach.
But it has two disadvantages:
1. For architecture-specific code that _always_ runs, such as the
MIPS32r2 implementation of chacha, the compiler no longer has an
opportunity to remove the generic code entirely from the binary,
which under Zinc resulted in a smaller module.
2. The inliner can't make optimizations for that call.
Disadvantage (2) might not make much of a difference. Disadvantage (1)
seems like a bigger deal. However, perhaps the linker is smart and can
remove the code and symbol? Or if not, is there a way to make the linker
smart? Or would all this require crazy LTO which isn't going to happen
any time soon?
