Re: [RFC PATCH 00/13] RISC-V crypto with reworked asm files

[Eric Biggers]

On Wed, Jan 03, 2024 at 03:00:29PM +0100, Ard Biesheuvel wrote:
> On Tue, 2 Jan 2024 at 07:50, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> >
> > As discussed previously, the proposed use of the so-called perlasm for
> > the RISC-V crypto assembly files makes them difficult to read, and these
> > files have some other issues such extensively duplicating source code
> > for the different AES key lengths and for the unrolled hash functions.
> > There is/was a desire to share code with OpenSSL, but many of the files
> > have already diverged significantly; also, for most of the algorithms
> > the source code can be quite short anyway, due to the native support for
> > them in the RISC-V vector crypto extensions combined with the way the
> > RISC-V vector extension naturally scales to arbitrary vector lengths.
> >
> > Since we're still waiting for prerequisite patches to be merged anyway,
> > we have a bit more time to get this cleaned up properly.  So I've had a
> > go at cleaning up the patchset to use standard .S files, with the code
> > duplication fixed.  I also made some tweaks to make the different
> > algorithms consistent with each other and with what exists in the kernel
> > already for other architectures, and tried to improve comments.
> >
> > The result is this series, which passes all tests and is about 2400
> > lines shorter than the latest version with the perlasm
> > (https://lore.kernel.org/linux-crypto/20231231152743.6304-1-jerry.shih@xxxxxxxxxx/).
> > All the same functionality and general optimizations are still included,
> > except for some minor optimizations in XTS that I dropped since it's not
> > clear they are worth the complexity.  (Note that almost all users of XTS
> > in the kernel only use it with block-aligned messages, so it's not very
> > important to optimize the ciphertext stealing case.)
> >
> > I'd appreciate people's thoughts on this series.  Jerry, I hope I'm not
> > stepping on your toes too much here, but I think there are some big
> > improvements here.
> >
> 
> As I have indicated before, I fully agree with Eric here that avoiding
> perlasm is preferable: sharing code with OpenSSL is great if we can
> simply adopt the exact same code (and track OpenSSL as its upstream)
> but this never really worked that well for skciphers, due to API
> differences. (The SHA transforms can be reused a bit more easily)
> 
> I will also note that perlasm is not as useful for RISC-V as it is for
> other architectures: in OpenSSL, perlasm is also used to abstract
> differences in calling conventions between, e.g., x86_64 on Linux vs
> Windows, or to support building with outdated [proprietary]
> toolchains.
> 
> I do wonder if we could also use .req directives for register aliases
> instead of CPP defines? It shouldn't matter for working code, but the
> diagnostics tend to be a bit more useful if the aliases are visible to
> the assembler.

.req unfortunately isn't an option since it is specific to AArch64 and ARM
assembly.  So we have to use #defines like x86 does.  Ultimately, the effect is
about the same.

- Eric