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
