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

[Andrew Jones]

On Wed, Aug 30, 2023 at 10:33:04AM -0700, Evan Green wrote:
> On Wed, Aug 30, 2023 at 2:03 AM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Aug 29, 2023 at 10:20:04AM -0700, Evan Green wrote:
> > > On Tue, Aug 29, 2023 at 1:48 AM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > Hi Evan,
> > > >
> > > > Here's my stab at new wording.
> > > >
> > > > On Tue, Jul 11, 2023 at 01:18:30PM -0700, Evan Green wrote:
> > > > ...
> > > > > 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.
> > > > > +
> > > >
> > > > The "isa" and "hart isa" lines in /proc/cpuinfo list RISC-V ISA extensions
> > > > which the kernel can identify (the kernel recognizes the extension's name)
> > > > and have not been filtered out due to effectively not being present.  An
> > > > extension is effectively not present when it is unusable, either due to
> > > > defects (which the kernel is aware of), due to missing information which
> > > > is necessary to complete the extension's description, or due to being
> > > > explicitly "hidden", such as when a kernel command line parameter
> > > > instructs the kernel to pretend the extension is not present.  Note, an
> > > > extension's presence in a list does not imply the kernel is using the
> > > > extension, nor does it imply that userspace or guest kernels may use the
> > > > extension (__riscv_hwprobe() should be queried for userspace usability.
> > > > The hypervisor should be queried, using hypervisor-specific APIs, to
> > > > check guest kernel usability.)
> > > >
> > > > The "isa" line describes the lowest common denominator of extensions,
> > > > which are the extensions implemented on all harts.  In contrast, the
> > > > extensions listed in the "hart isa" line need not be implemented by
> > > > any other hart than the hart corresponding to the line.
> > > >
> > > > ---
> > > >
> > > > I've specifically dropped the 'The "hart isa" line is consistent with
> > > > what's returned by __riscv_hwprobe()...' part because I suspect hwprobe
> > > > could at least lag what gets put in "hart isa", since the kernel may be
> > > > taught about an extension for a different purpose first, neglecting
> > > > hwprobe. And, there may be cases that hwprobe never enumerates an
> > > > extension which the kernel does.
> > >
> > > Thanks for this. My v5 had also dropped the hwprobe part. A few thoughts:
> > >
> > >  * It seems like you want to make sure we call out the fact that the
> > > kernel may trim out, for various reasons, ISA extensions that the
> > > hardware does in fact support. This seems reasonable, but I don't
> > > think we need to enumerate the complete list of "why" this might
> > > happen, as that list is likely to go stale.
> >
> > I agree it's better to not [try to] list all the possibilities, assuming
> > we can come up with good, general words to capture the idea.
> >
> > >  * The "kernel is using the extension" part is a slightly confusing
> > > perspective in this context, as it sort of implies the kernel has its
> > > own agenda :). I'd expect users looking at /proc/cpuinfo are mostly
> > > trying to figure out what extensions they themselves can use, and the
> > > kernel's behavior factors in only insofar as it's required to support
> > > the user in using a feature. Mostly I guess this is a phrasing nit.
> >
> > We'll have plenty of S-mode extensions listed in these strings. Users
> > who recognize S-mode extensions may want to know if they're listed because
> > the kernel is applying them (and wouldn't be listed otherwise), or whether
> > they're listed simply because they exist on the hart(s).
> 
> I see. You're right I was thinking only about U-mode extensions.
> 
> >
> > >  * The bringing up of guest kernels also seems confusing to me in the
> > > context of /proc/cpuinfo. I'd expect discussions on how host ISA
> > > extensions filter into guest OSes to be in a hypervisor-specifc
> > > document, or at least a section dedicated to virtualization.
> >
> > If there weren't S-mode extensions being listed, then I would agree,
> > but, since there are, it seems odd to not explain what it means for
> > them to be there wrt host and guest kernels.
> 
> I'm not a virtualization guy, but my impression was people didn't have
> expectations that everything they saw in cpuinfo would be wholesale
> presented to guest VMs. There's always that layer of hypervisor
> configuration that may strip out some features. So I'm still not super
> convinced guest VMs need a carveout/caveat here, but let me see if I
> can fold it in.
> 
> >
> > >  * I hesitated in adding prescriptive guidance on what users should
> > > do, as I think this section will hold up better over time if it just
> > > describes current characteristics, rather than attempting to prescribe
> > > behavior. If we want a prescriptive documentation on "use this for
> > > that", that should probably be its own section
> >
> > I guess the guidance you're referring to is the "(__riscv_hwprobe() should
> > be queried for userspace usability.  The hypervisor should be queried,
> > using hypervisor-specific APIs, to check guest kernel usability.)" bit.
> > I'm fine with dropping that or moving it to another section, but I think
> > the more we point out hwprobe, the better. If developers are reading this
> > /proc/cpuinfo section because they want to detect extensions, then I'd
> > prefer the section redirects them to hwprobe.
> 
> Fair enough. I still haven't brought myself to wedge in an ad for
> hwprobe, but I also don't disagree with this :)
> 
> >
> > >
> > > If I try to fold the gist of what you wrote into v5, I get something
> > > like this (also with a very slight section heading change). Let me
> > > know what you think:
> > >
> > > "isa" and "hart isa" lines in /proc/cpuinfo
> > > ------------------------------------------
> >
> > need one more _
> >
> > >
> > > The "isa" line in /proc/cpuinfo describes the lowest common denominator of
> > > RISC-V ISA extensions recognized by the kernel and implemented on all harts. The
> > > "hart isa" line, in contrast, describes the set of extensions recognized by the
> > > kernel on the particular hart being described, even if those extensions may not
> > > be present on all harts in the system.
> > >
> > > In both lines, the presence of an extension guarantees only that the
> > > hardware has the described capability.
> > > Additional kernel support or policy changes may be required before an
> > > extension's capability is fully usable by userspace programs.
> >
> > or guest kernels in the case of S-mode extensions.
> >
> > >
> > > Inversely, the absence of an extension in these lines does not
> > > necessarily mean the hardware does not support that feature. The
> > > running kernel may not recognize the extension, or may have
> > > deliberately disabled access to it.
> >
> > I'm not sure about the word "disabled". The kernel can only disable U-mode
> > extensions and S-mode extensions for guests. S-mode extensions for the
> > current kernel would have to be disabled by its next higher privilege
> > level.
> >
> > How about "...may not recognize the extension, or may have deliberately
> > removed it from the listing."
> 
> Perfect.
> 
> >
> > (But then readers will wonder why an extension would be deliberately
> > removed from the listing, which brings us back to trying to come up
> > with general words to capture the cases I listed. Or, maybe we don't
> > have to care if they wonder why in this section/document.)
> 
> I know, I felt the same pull to explain why as well. But I think given
> that our goal with this section is mostly to make the distinction of
> "presence != active", explaining exactly why the kernel may remove it
> is not strictly necessary. All of my attempts to tack something on end
> up being enumerated lists, which don't come out well and muddle the
> message.
> 
> Here's another shot (line endings may be wonky):
> 
> "isa" and "hart isa" lines in /proc/cpuinfo
> -------------------------------------------
> 
> The "isa" line in /proc/cpuinfo describes the lowest common denominator of
> RISC-V ISA extensions recognized by the kernel and implemented on all harts. The
> "hart isa" line, in contrast, describes the set of extensions recognized by the
> kernel on the particular hart being described, even if those extensions may not
> be present on all harts in the system.
> 
> In both lines, the presence of an extension guarantees only that the
> hardware has the described capability.
> Additional kernel support or policy changes may be required before an
> extension's capability is fully usable by userspace programs.
> Similarly, for S-mode extensions, presence in one of these lines does
> not guarantee that the kernel is taking advantage of the extension, or
> that the feature will be visible in guest VMs managed by this kernel.
> 
> Inversely, the absence of an extension in these lines does not
> necessarily mean the hardware does not support that feature. The
> running kernel may not recognize the extension, or may have
> deliberately removed it from the listing.

Looks good to me!

Thanks,
drew