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: > > > > In /proc/cpuinfo, most of the information we show for each processor is > > > > specific to that hart: marchid, mvendorid, mimpid, processor, hart, > > > > compatible, and the mmu size. But the ISA string gets filtered through a > > > > lowest common denominator mask, so that if one CPU is missing an ISA > > > > extension, no CPUs will show it. > > > > > > > > Now that we track the ISA extensions for each hart, let's report ISA > > > > extension info accurately per-hart in /proc/cpuinfo. We cannot change > > > > the "isa:" line, as usermode may be relying on that line to show only > > > > the common set of extensions supported across all harts. Add a new "hart > > > > isa" line instead, which reports the true set of extensions for that > > > > hart. This matches what is returned in riscv_hwprobe() when querying a > > > > given hart. > > > > > > > > Signed-off-by: Evan Green <evan@xxxxxxxxxxxx> > > > > Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx> > > > > Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx> > > > > > > > > --- > > > > > > > > Changes in v4: > > > > - Documentation: Made the underline match the text line (Conor) > > > > - Documentation: hanged "in question" to "being described" (Andrew) > > > > > > > > Changes in v3: > > > > - Add some documentation (Conor) > > > > > > > > Changes in v2: > > > > - Added new "hart isa" line rather than altering behavior of existing > > > > "isa" line (Conor, Palmer) > > > > > > > > > > > > I based this series on top of Conor's riscv-extensions-strings branch > > > > from July 3rd, since otherwise this change gets hopelessly entangled > > > > with that series. > > > > > > > > --- > > > > Documentation/riscv/uabi.rst | 10 ++++++++++ > > > > arch/riscv/kernel/cpu.c | 22 ++++++++++++++++++---- > > > > 2 files changed, 28 insertions(+), 4 deletions(-) > > > > > > > > diff --git a/Documentation/riscv/uabi.rst b/Documentation/riscv/uabi.rst > > > > index 8960fac42c40..afdda580e5a2 100644 > > > > --- a/Documentation/riscv/uabi.rst > > > > +++ b/Documentation/riscv/uabi.rst > > > > @@ -42,6 +42,16 @@ An example string following the order is:: > > > > > > > > rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux > > > > > > > > +"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. -Evan
