On Mon, Jan 06, 2025 at 11:12:53AM +0000, Marc Zyngier wrote: > On Mon, 06 Jan 2025 09:40:56 +0000, > Mark Rutland <mark.rutland@xxxxxxx> wrote: > > > > On Fri, Jan 03, 2025 at 06:22:55PM +0000, Marc Zyngier wrote: > > > The hwcaps code that exposes SVE features to userspace only > > > considers ID_AA64ZFR0_EL1, while this is only valid when > > > ID_AA64PFR0_EL1.SVE advertises that SVE is actually supported. > > > > > > The expectations are that when ID_AA64PFR0_EL1.SVE is 0, the > > > ID_AA64ZFR0_EL1 register is also 0. So far, so good. > > > > > > Things become a bit more interesting if the HW implements SME. > > > In this case, a few ID_AA64ZFR0_EL1 fields indicate *SME* > > > features. And these fields overlap with their SVE interpretations. > > > But the architecture says that the SME and SVE feature sets must > > > match, so we're still hunky-dory. > > > > > > This goes wrong if the HW implements SME, but not SVE. In this > > > case, we end-up advertising some SVE features to userspace, even > > > if the HW has none. That's because we never consider whether SVE > > > is actually implemented. Oh well. > > > > Ugh; this is a massive pain. :( > > > > Was this found by inspection, or is some real software going wrong? > > Catalin can comment on that -- I understand that he found existing SW > latching on SVE2 being wrongly advertised as hwcaps. > > > > Fix it by restricting all SVE capabilities to ID_AA64PFR0_EL1.SVE > > > being non-zero. > > > > Unfortunately, I'm not sure this fix is correct+complete. > > > > We expose ID_AA64PFR0_EL1 and ID_AA64ZFR0_EL1 via ID register emulation, > > so any userspace software reading ID_AA64ZFR0_EL1 will encounter the > > same surprise. If we hide that I'm worried we might hide some SME-only > > information that isn't exposed elsewhere, and I'm not sure we can > > reasonably hide ID_AA64ZFR0_EL1 emulation for SME-only (more on that > > below). > > I don't understand where things go wrong. EL0 SW that looks at the ID > registers should perform similar checks, and we are not trying to make > things better on that front (we can't). Unless you invent time travel > and fix the architecture 5 years ago... :-/ Fair enough; if we say software consuming ID_AA64ZFR0_EL1 must check ID_AA64PFR0_EL1.SVE or ID_AA64PFR1_EL1.SME first, and we leave the emulation of ID_AA64ZFR0_EL1 as-is, that's fine by me. > The hwcaps are effectively demultiplexing the ID registers, and they > have to be exact, which is what this patch provides (SVE2 doesn't get > wrongly advertised when not present). > > Secondly, all our HWCAP documentation is written in the form: > > > > | HWCAP2_SVEBF16 > > | Functionality implied by ID_AA64ZFR0_EL1.BF16 == 0b0001. > > > > ... so while the architectural behaviour is a surprise, the kernel is > > (techincallyy) behaving exactly as documented prior to this patch. Maybe > > we need to change that documentation? > > Again, I don't see what goes wrong here. BF16 is only implemented for > SVE or SME+FA64, and FA64 requires SVE2. So at least for that one, we > should be good. That was probably a bad example. What I was trying to get at is that the HWCAPs are behavind exactly *as documented*, but that's not what we actually want them to describe. For example, SVE2 is described as: | Functionality implied by ID_AA64ZFR0_EL1.SVEver == 0b0001. ... which is exactly what we check today, but that doesn't architecturally imply FEAT_SVE2 on SME-only HW where it can apparently be 0b0001 due to FEAT_SME alone. So to match the code change we'd need to change that to something like: | Functionality impled by ID_AA64PFR0_EL1 == 0b0001 and | ID_AA64ZFR0_EL1.SVEver == 0b0001 ... with similar for other hwcaps. > > Do we have equivalent SME hwcaps for the relevant features? > > > > ... looking at: > > > > https://developer.arm.com/documentation/ddi0601/2024-12/AArch64-Registers/ID-AA64ZFR0-EL1--SVE-Feature-ID-Register-0?lang=en > > > > ... I see that ID_AA64ZFR0_EL1.B16B16 >= 0b0010 implies the presence of > > SME BFMUL and BFSCALE instructions, but I don't see something equivalent > > in ID_AA64SMFR0_EL1 per: > > > > https://developer.arm.com/documentation/ddi0601/2024-12/AArch64-Registers/ID-AA64SMFR0-EL1--SME-Feature-ID-Register-0?lang=en > > > > ... so I suspect ID_AA64ZFR0_EL1 might be the only source of truth for > > this. > > Indeed, and the SME HWCAPs are not doing the right thing either. Or > rather, we have no way to tell userspace that BFMUL/BFSCALE are > available. To be clear, I'm happy to punt on adding SME-specific HWCAPs, I just want to make sure we're agreed as to whether the existing HWCAPs should be SVE-specific, which it sounds like we are? Mark.
