Re: [PATCH v2 4/6] RISC-V: Detect unaligned vector accesses supported.

Conor Dooley
2024-06-21T18:03:01Z

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On Fri, Jun 21, 2024 at 10:58:16AM -0700, Eric Biggers wrote:
> On Fri, Jun 21, 2024 at 10:18:23AM -0700, Charlie Jenkins wrote:
> > > Additionally, what are we doing in the kernel if we detect that
> > > misaligned stuff isn't supported? Are we going to mandate that kernel
> > > code is aligned only, disable in-kernel vector or some other mechanism
> > > to make sure that things like crypto code don't have/introduce code
> > > that'll not run on these systems?
> >
> > UNSUPPORTED will still be set by the quick probe so it would be possible
> > for the kernel/userspace to avoid running misaligned vector when it's
> > unsupported. Any kernel methods would probably want to always run
> > aligned vector unless misaligned support was determined to be FAST
> > anyway, I am doubtful that code will have different optimizations for
> > FAST, SLOW, and UNSUPPORTED but it is possible.
> >
> > I would prefer consistency between scalar and vector misaligned support,
> > but this is not a deal breaker for this patch. I am not convinced it is
> > the best choice, but I am okay with leaving this option in the kernel.
> >
>
> Note that most of the vector crypto code (in arch/riscv/crypto/) assumes that
> vector misaligned accesses are supported. Many of the RISC-V vector crypto
> instructions require using SEW=32 or SEW=64, and as a result, loads and stores
> of data can be misaligned unless the code changes the SEW to 8 and back again,
> which would be inefficient and add extra complexity. I don't anticipate
> workarounds for CPUs that couldn't be bothered to support misaligned accesses
> being added.

> So what we'll probably have to do is just disable the vector
> crypto algorithms if the CPU doesn't support misaligned accesses...

Right. I was thinking similarly, and that we should just disable all
in-kernel vector code if the platform doesn't support misaligned vector.

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