Re: [PATCH v4] RISC-V: Show accurate per-hart isa in /proc/cpuinfo

[Conor Dooley]

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
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...

> 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".

Thanks,
Conor.

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