Re: [PATCH v5] RISC-V: Show accurate per-hart isa in /proc/cpuinfo

[Conor Dooley]

On Mon, Aug 28, 2023 at 10:18:24AM -0700, Evan Green wrote:
> On Mon, Aug 28, 2023 at 9:53 AM Conor Dooley <conor@xxxxxxxxxx> wrote:
> >
> > On Mon, Aug 28, 2023 at 09:24:03AM -0700, Evan Green wrote:
> > > On Sat, Aug 26, 2023 at 2:56 AM Conor Dooley <conor@xxxxxxxxxx> wrote:
> > > >
> > > > On Sat, Aug 26, 2023 at 12:26:25AM +0100, Conor Dooley wrote:
> > > > > On Fri, Aug 25, 2023 at 04:11:38PM -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.
> > > > > >
> > > > > > Signed-off-by: Evan Green <evan@xxxxxxxxxxxx>
> > > > > > Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> > > > >
> > > > > > Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> > > > >
> > > > > Can you drop this if you repost?
> > >
> > > Will do.
> > >
> > > > >
> > > > > > +"isa" vs "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 cases, the presence of a feature
> > > > > > +in these lines guarantees only that the hardware has the described capability.
> > > > > > +Additional kernel support or policy control changes may be required before a
> > > > > > +feature is fully usable by userspace programs.
> > > > >
> > > > > I do not think that "in both cases" matches the expectations of
> > > > > userspace for the existing line. It's too late at night for me to think
> > > > > properly, but I think our existing implementation does work like you
> > > > > have documented for FD/V. I think I previously mentioned that it could
> > > > > misreport things for vector during the review of the vector series but
> > > > > forgot about it until now.
> > > >
> > > > I went and checked, and yes it does currently do that for vector. I
> > > > don't think that that is what userspace would expect, that Google
> > > > cpu_features project for example would draw incorrect conclusions.
> > >
> > > I'm lost, could you explain a little more?
> >
> > There (may be/)are userspace programs that will interpret the appearance
> > of extensions in cpuinfo as meaning they can be used without performing
> > any further checks.
> >
> > > My goal was to say that
> > > there's no blanket guarantee that the feature is 100% ready to go for
> > > userspace just because it's seen here.
> >
> > Right. I was agreeing that this is true, but it is also not how some
> > userspace programs have interpreted things. Consider a platform & kernel
> > that support the V extension but vector has not been enabled by default
> > or by early userspace. If someone cats cpuinfo, they'll see v there, but
> > it won't be usable. That Google cpu_features project (or a punter) may
> > then assume they can use it, as that's been the case so far in general*.
> >
> > The caveat producing the * being that the same problem actually exists
> > for F/D too AFAICT, but it's likely that nobody really encountered it
> > as they didn't build non-FP userspaces & then try to use FP in some
> > userspace applications.
> >
> > > For some extensions, it may in
> > > fact end up meaning just that (hence the "additional ... may be
> > > required" rather than "is required").
> >
> > > This is true for FD (maybe,
> > > depending on history?),
> >
> > AFAICT, it's not true for FD. The FPU config option not being set, or
> > either of F and D being missing will lead to unusable extensions
> > appearing.
> 
> Ah ok.
> 
> >
> > > or extensions whose minimal/zero kernel
> > > support was unconditionally added at the same time as its parsing for
> > > it. But it's not true solely by virtue of being in /proc/cpuinfo. In
> > > other words, I'm trying to establish the floor of what /proc/cpuinfo
> > > guarantees, without fully specifying the ceiling.
> >
> > > Are you saying that
> > > we need to spell out the guarantees for each extension?
> >
> > No, I don't want that!
> >
> > > Or are you
> > > saying the floor I've defined in general is incorrect or insufficient?
> >
> > I think the floor that you have defined is probably misleading to users.
> > It's also the floor that has existed for quite a while, so this might be
> > a case of the userspace devs messing up due to an absence of any
> > explanation of what to do here.
> > Things will get abhorrently messy if we try to do what these userspace
> > programs expect, and I don't think we should go there. We just need to
> > bear in mind that the behaviour we have & the behaviour that you are
> > documenting flys in the face of what some userspace expects.
> 
> Thanks, I think I understand now. You're saying the floor I'm defining
> might surprise some users, who were expecting the floor to be "fully
> enabled and ready to party".

Yes.

> Given there was no documentation about it
> before, and this documentation is consistent with what we actually do
> (and there seems to be consensus this is a maintainable position to
> hold), can we just tell those users they're holding it wrong?

I think we have to. Dealing with the vector prctls & sysctls here would
be horrid, as I discussed with Andy on one of the versions of that
series.

> > > I'm also open to direct suggestions of wording if you've got something
> > > in mind :)
> >
> > Someone mentioned it recently, but it really is starting to feel more
> > and more like lscpu should grow support for hwprobe and funnel people
> > into using that instead of /proc/cpuinfo when all they want is to see
> > what their hardware can do.
> 
> Maybe for the fiddly microarchitectural bits, yeah. But I'd think our
> newly proposed documentation for /proc/cpuinfo of keeping it closer to
> what the hardware can do would suit the lscpu folks' mission well. (In
> ChromeOS at least, we didn't have lscpu, but snarfed /proc/cpuinfo
> directly into feedback reports that consented to sending along system
> info). Really I'd think it's the application/library writers who want
> to know "am I ready to go right now" are who we should be pushing to
> use hwprobe, since we can define those bits to be as specific as we
> want (eg V is on AND it's a full moon, so go for it).
> 
> Depending on your thoughts on this, if there are changes requested on
> this patch, let me know what they are.

Nah, I think it's fine & the r-b I left on the previous version can stay
:)

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