On Wed, Nov 04, 2020 at 06:49:05PM +0000, Mark Brown wrote: > On Wed, Nov 04, 2020 at 06:13:06PM +0000, Dave Martin wrote: > > On Wed, Nov 04, 2020 at 05:50:33PM +0000, Mark Brown wrote: > > > > 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 > > ... > > > 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. > > I agree, that's most likely a good default. > > > 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. > > I think a global control would be good for testing, it seems simpler and > easier all round. The per thing tuning seems more useful for cases > where we run into something like a performance reason to use a limited > set of vector lengths but I think we should only add that when we have > at least one user for it, some examples of actual restrictions we want > would probably be helpful for designing the interface. Ack; note that an algo that wants to use a particular vector length can do so by means of the special predicate patterns VLnnn, POW2, MUL3 etc. So setting an explicit limit in ZCR_EL1.LEN should hopefully be an uncommon requirement. > > > 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. > > Definitely worth exploring. Cheers ---Dave
