On Thu, Aug 24, 2023 at 03:06:53PM -0700, Evan Green wrote: > On Thu, Aug 24, 2023 at 10:29 AM Conor Dooley <conor@xxxxxxxxxx> wrote: > > On Thu, Aug 24, 2023 at 09:18:16AM -0700, Evan Green wrote: > > > On Thu, Aug 24, 2023 at 5:20 AM Conor Dooley <conor.dooley@xxxxxxxxxxxxx> wrote: > > > > On Tue, Jul 11, 2023 at 01:18:30PM -0700, Evan Green wrote: > > > > > +"isa" vs "hart isa" lines in /proc/cpuinfo > > > > > +------------------------------------------ > > > > > + > > > > > +The "isa" line in /proc/cpuinfo describes the lowest common denominator of > > > > > +RISC-V ISA extensions understood by the kernel and implemented on all harts. The > > > > > +"hart isa" line, in contrast, describes the set of extensions understood by the > > > > > +kernel on the particular hart being described, even if those extensions may not > > > > > +be present on all harts in the system. The "hart isa" line is consistent with > > > > > +what's returned by __riscv_hwprobe() when querying for that specific CPU. > > > > > > > > Thinking about this again, I don't think this is true. hwprobe uses > > > > has_fpu(), has_vector() etc that interact with Kconfig options but the > > > > percpu isa bitmap isn't affected by these. > > > > > > Ugh yeah it's kind of a mishmash isn't it. hwprobe_isa_ext0() uses the > > > lowest common denominator for FD, C, V, but per-hart info for > > > Zba,Zbb,Zbs. Given the interface, per-hart info seems like what we > > > should have done there, and the FD, C, and V were my bad. The good > > > news is we can define new bits that do the right thing, though maybe > > > we should wait until someone actually wants them. For this patch we > > > should just remove this sentence. We can also correct the > > > documentation in hwprobe to mention the shortcoming in FD,C,V. > > > > I'm not really sure it's all that much of a shortcoming for V or FD, > > since without the kernel support you shouldn't be using those extensions > > anyway. A hwprobe thing for that sounds like a footgun to me & I think > > the current behaviour is how it should be for these extensions. > > It not being per-cpu is arguably a bug I suppose? But I would contend > > Yeah it was mostly the not being per-cpu I was pointing to in my previous email. > > > that we are conveying support for the extension on a per-hart level, > > with it then also gated by the kernel supporting V or FD, which is on a > > system-wide basis. > > Any other extensions that require Kconfig-gated kernel support should > > also not report via hwprobe that the extension is supported when the > > Kconfig option is disabled. It just so happens that the set of > > extensions that hwprobe supports that are Kconfig-gated and those that > > require all-hart support are one and the same right now, so we can kinda > > just conflate the two & use has_vector() et al that handles both > > kconfig-gating and all-hart support. Until something comes along that needs > > anything different, I'd leave well enough alone for hwprobe... > > Sounds good. > > > > > > Palmer, do you want a spin of this patch or a followup on top to > > > remove the above sentence? > > > > It's not actually been applied yet, right? > > > > Do you want to have this new thing in cpuinfo tell the user "this hart > > has xyz extensions that are supported by a kernel, but maybe not this > > kernel" or to tell the user "this hart has xyz extensions that are > > supported by this kernel"? Your text above says "understood by the > > kernel", but I think that's a poor definition that needs to be improved > > to spell out exactly what you mean. IOW does "understood" mean the > > kernel will parse them into a structure, or does it mean "yes you can > > use this extension on this particular hart". > > I'm imagining /proc/cpuinfo being closer to "the CPU has it and the > kernel at least vaguely understands it, but may not have full support > for it enabled". I'm assuming /proc/cpuinfo is mostly used by 1) > humans wanting to know if they have hardware support for a feature, > and 2) administrators collecting telemetry to manage fleets (ie do I > have any hardware deployed that supports X). Programmers looking to > see "is the kernel support for this feature ready right now" would > ideally not be parsing /proc/cpuinfo text, as more direct mechanisms > like specific hwprobe bits for "am I fully ready to go" would be > easier to work with. Feel free to yell at me if this overall vision > seems flawed. > > I tried to look to see if there was consensus among the other > architectures. Aarch64 seems to go with "supported and fully enabled", > as their cpu_has_feature() directly tests elf_hwcap, and elements in > arm64_elf_hwcaps[] are Kconfig gated. X86 is complicated, but IIRC is > more along the lines of "hardware has it". They have two macros, > cpu_has() for "raw capability" and cpu_feature_enabled() for "kernel > can do it too", and they use cpu_has() for /proc/cpuinfo flags. I'm fine with the per-cpu stuff meaning "the hardware has it and a kernel, but not necessarily this one, supports it" - just please make the documentation clear about it.
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