On Tue, Mar 18, 2025 at 03:09:18PM +0100, Alexandre Ghiti wrote: > On 18/03/2025 14:04, Andrew Jones wrote: > > On Tue, Mar 18, 2025 at 01:58:10PM +0100, Alexandre Ghiti wrote: > > > On 18/03/2025 13:45, Andrew Jones wrote: > > > > On Tue, Mar 18, 2025 at 01:13:18PM +0100, Alexandre Ghiti wrote: > > > > > On 18/03/2025 09:48, Andrew Jones wrote: > > > > > > On Mon, Mar 17, 2025 at 03:39:01PM +0100, Alexandre Ghiti wrote: > > > > > > > Hi Drew, > > > > > > > > > > > > > > On 04/03/2025 13:00, Andrew Jones wrote: > > > > > > > > Allow skipping scalar and vector unaligned access speed tests. This > > > > > > > > is useful for testing alternative code paths and to skip the tests in > > > > > > > > environments where they run too slowly. All CPUs must have the same > > > > > > > > unaligned access speed. > > > > > > > I'm not a big fan of the command line parameter, this is not where we should > > > > > > > push uarch decisions because there could be many other in the future, the > > > > > > > best solution to me should be in DT/ACPI and since the DT folks, according > > > > > > > to Palmer, shut down this solution, it remains using an extension. > > > > > > > > > > > > > > I have been reading a bit about unaligned accesses. Zicclsm was described as > > > > > > > "Even though mandated, misaligned loads and stores might execute extremely > > > > > > > slowly. Standard software distributions should assume their existence only > > > > > > > for correctness, not for performance." in rva20/22 but *not* in rva23. So > > > > > > > what about using this "hole" and consider that a platform that *advertises* > > > > > > > Zicclsm means its unaligned accesses are fast? After internal discussion, It > > > > > > > actually does not make sense to advertise Zicclsm if the platform accesses > > > > > > > are slow right? > > > > > > This topic pops up every so often, including in yesterday's server > > > > > > platform TG call. In that call, and, afaict, every other time it has > > > > > > popped up, the result is to reiterate that ISA extensions never say > > > > > > anything about performance. So, Zicclsm will never mean fast and we > > > > > > won't likely be able to add any extension that does. > > > > > Ok, I should not say "fast". Usually, when an extension is advertised by a > > > > > platform, we don't question its speed (zicboz, zicbom...etc), we simply use > > > > > it and it's up to the vendor to benchmark its implementation and act > > > > > accordingly (i.e. do not set it in the isa string). > > > > > > > > > > > > > > > > > arm64 for example considers that armv8 has fast unaligned accesses and can > > > > > > > then enable HAVE_EFFICIENT_ALIGNED_ACCESS in the kernel, even though some > > > > > > > uarchs are slow. Distros will very likely use rva23 as baseline so they will > > > > > > > enable Zicclsm which would allow us to take advantage of this too, without > > > > > > > this, we lose a lot of perf improvement in the kernel, see > > > > > > > https://lore.kernel.org/lkml/20231225044207.3821-1-jszhang@xxxxxxxxxx/. > > > > > > > > > > > > > > Or we could have a new named feature for this, even though it's weird to > > > > > > > have a named feature which would basically mean "Zicclsm is fast". We don't > > > > > > > have, for example, a named feature to say "Zicboz is fast" but given the > > > > > > > vague wording in the profile spec, maybe we can ask for one in that case? > > > > > > > > > > > > > > Sorry for the late review and for triggering this debate... > > > > > > No problem, let's try to pick the best option. I'll try listing all the > > > > > > options and there pros/cons. > > > > > > > > > > > > 1. Leave as is, which is to always probe > > > > > > pro: Nothing to do > > > > > > con: Not ideal in all environments > > > > > > > > > > > > 2. New DT/ACPI description > > > > > > pro: Describing whether or not misaligned accesses are implemented in > > > > > > HW (which presumably means fast) is something that should be done > > > > > > in HW descriptions > > > > > > con: We'll need to live with probing until we can get the descriptions > > > > > > defined, which may be never if there's too much opposition > > > > > > > > > > > > 3. Command line > > > > > > pro: Easy and serves its purpose, which is to skip probing in the > > > > > > environments where probing is not desired > > > > > > con: Yet another command line option (which we may want to deprecate > > > > > > someday) > > > > > > > > > > > > 4. New ISA extension > > > > > > pro: Easy to add to HW descriptions > > > > > > con: Not likely to get it through ratification > > > > > > > > > > > > 5. New SBI FWFT feature > > > > > > pro: Probably easier to get through ratification than an ISA extension > > > > > > con: Instead of probing, kernel would have to ask SBI -- would that > > > > > > even be faster? Will all the environments that want to skip > > > > > > probing even have a complete SBI? > > > > > > > > > > > > 6. ?? > > > > > So what about: > > > > > > > > > > 7. New enum value describing the performance as "FORCED" or "HW" (or > > > > > anything better) > > > > > pro: We only use the existing Zicclsm > > > > > con: It's not clear that the accesses are fast but it basically says to > > > > > SW "don't think too much, I'm telling you that you can use it", up to us to > > > > > describe this correctly for users to understand. > > > > But Zicclsm doesn't mean misaligned accesses are in HW, it just means > > > > they're not going to explode. > > > > > > They never explode since if they are not supported by the HW, we rely on > > > S-mode emulation already. > > Exactly. Zicclsm is just a new name for that behavior. Profiles try to > > name every behavior, even the ones we take for granted. Unfortunately, > > like in the case of Zicclsm, we don't necessarily gain anything from > > the new name. In this case, we don't gain a way to avoid probing. > > > I understand your point but given the misaligned traps exist, I can't find > another meaning to Zicclsm than "I'm telling you to use it". Zicclsm can't > be used to describe an OS behaviour (ie the emulation of misaligned > accesses). > > I'm also insisting because we need a compile-time hint which allows us to > enable HAVE_EFFICIENT_UNALIGNED_ACCESS in the kernel and Zicclsm is great > since it is required in RVA23. if that's not Zicclsm, that must be another > named feature/extension. > > What do you suggest to make progress here? > I guess you mean besides listing five options and posting patches for two of them :-) We can't force semantics onto Zicclsm and I doubt we'll get agreement to make another extension with the semantics we want. So (4) is out. I agree with Clement that (5) isn't good. That leaves (2). I guess we should start by trying to understand what issues there were/are with it. Thanks, drew
