Subject: Re: [PATCH 1/1] arm64: Accelerate Adler32 using arm64 SVE instructions.

From: Dave Martin <[email-masked]>
Date: Wed, 4 Nov 2020 18:13:06 +0000

On Wed, Nov 04, 2020 at 05:50:33PM +0000, Mark Brown wrote:
> On Tue, Nov 03, 2020 at 06:00:32PM +0000, Dave Martin wrote:
> > On Tue, Nov 03, 2020 at 03:34:27PM +0100, Ard Biesheuvel wrote:
> 
> > > First of all, I don't think it is safe at the moment to use SVE in the
> > > kernel, as we don't preserve all state IIRC. My memory is a bit hazy,
> 
> > I'm not convinced that it's safe right now.  SVE in the kernel is
> > unsupported, partly due to cost and partly due to the lack of a
> > compelling use case.
> 
> I think at a minimum we'd want to handle the vector length explicitly
> for kernel mode SVE, vector length independent code will work most of
> the time but at the very least it feels like a landmine waiting to cause
> trouble.  If nothing else there's probably going to be cases where it
> makes a difference for performance.  Other than that I'm not currently
> seeing any issues since we're handling SVE in the same paths we handle
> the rest of the FPSIMD stuff.

Having a random vector length could be good for testing ;)

I was tempted to add that as a deliberate feature, but that sort of
nothing doesn't really belong in the kernel...

Anyway:

The main reasons for constraining the vector length are a) to hide
mismatches between CPUs in heterogeneous systems, b) to ensure that
validated software doesn't run with a vector length it wasn't validated
for, and c) testing.

For kernel code, it's reasonable to say that all code should be vector-
length agnostic unless there's a really good reason not to be.  So we
may not care too much about (b).

In that case, just setting ZCR_EL1.LEN to max in kernel_sve_begin() (or
whatever) probably makes sense.

For (c), it might be useful to have a command-line parameter or debugfs
widget to constrain the vector length for kernel code; perhaps globally
or perhaps per driver or algo.

Otherwise, I agree that using SVE in the kernel _should_ probably work
safely, using the same basic mechanism as kernel_mode_neon().  Also,
it shouldn't have higher overheads than kernel-mode-NEON now.

> 
> > I think it would be preferable to see this algo accelerated for NEON
> > first, since all AArch64 hardware can benefit from that.
> 
> ...
> 
> > much of a problem.  kernel_neon_begin() may incur a save of the full SVE
> > state anyway, so in some ways it would be a good thing if we could
> > actually make use of all those registers.
> 
> > SVE hardware remains rare, so as a general policy I don't think we
> > should accept SVE implementations of any algorithm that does not
> > already have a NEON implementation -- unless the contributor can
> > explain why nobody with non-SVE hardware is going to care about the
> > performance of that algo.
> 
> I tend to agree here, my concerns are around the cost of maintaining a
> SVE implementation relative to the number of users who'd benefit from it
> rather than around the basic idea of using SVE at all.  If we were
> seeing substantial performance benefits over an implementation using
> NEON, or had some other strong push to use SVE like a really solid
> understanding of why SVE is a good fit for the algorithm but NEON isn't,
> then it'd be worth finishing up the infrastructure.  The infrastructure
> itself doesn't seem fundamentally problematic.

Agreed

Nonetheless, working up a candidate algorithm to help us see whether
there is a good use case seems like a worthwhile project, so I don't
want to discourage that too much.

Cheers
---Dave

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