Re: Vector on Banana Pi BPI-F3

[Conor Dooley <conor@kernel.org>]

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On Mon, Jul 01, 2024 at 03:19:51PM +0200, Robbin Ehn wrote:
> On Mon, Jul 1, 2024 at 2:09 PM Conor Dooley <conor@kernel.org> wrote:
> > On Mon, Jul 01, 2024 at 11:08:31AM +0200, Robbin Ehn wrote:
> > > There seems to be a large interest in running openjdk with vector on
> > > this board out-of-the-box.
> > > The vendor kernel have vector from 6.4 back-ported without hwprobe.
> > > And FYI: the vendor kernel for k230 has THEAD vector patches.
> > > Both do the right thing during context switches (sources have been checked).
> > >
> > > It seems like we have three options, anymore?
> > >
> > > A:
> > > Keep what we do: i.e. no hwprobe no V.
> > > Downside no V (default) on contemporary boards.
> > > Hence no V coverage in CI testing and disappointed early adopters.
> >
> > Upside: pressure on vendors not to ship old, heavily modified kernels.
> > I'd be pretty vociferous about pushing the blame on the vendors were I
> > in your shoes! There's some work in progress for mainline spacemit k1
> > support that would allow you to get something usable for CI although
> > obviously these things move slowly. Palmer did mention that if this
> > board is helpful for compiler testing that some of the Rivos folks might
> > work on support for it in mainline though, which would be neat...
> 
> Yes, I know about the mainline effort, I didn't mention that because
> if that work is not picked up by the vendor, most will probably stick
> with the vendor kernel.

Right. I meant that specifically on the CI point.

> > > B:
> > > Enable it for triplet mvendorid/marchid/mimplid.
> > > Only V for this specific board, other boards and/or updates to this
> > > board require new binaries.
> >
> > And, depending on implementation, may break if the kernel running on the
> > board doesn't support vector. You'd need to check hwcap and the triplet.
> > m*id all come from the CPU, so you can't be sure that the values there
> > will uniquely identify a specific board or SoC for you anyway. If you
> > want to support the can of worms you're opening there with m*id stuff
> > for vendor kernels, then go for it. If you start it for vector on one
> > board, where do you draw a line in terms of either extension or boards?
> >
> > > C:
> > > Enable it for boards with V in hwcap.
> > > Downside boards false reporting V (1.0), such as K230 small-CPU and THEAD,
> >
> > Why does the small K230 CPU falsely report that? Does it have no vector
> > and report that it does? Or does it support 0.7 like the other T-head
> > stuff? I suppose the latter given they the patches you say their kernel
> > has.
> 
> If you are running on the small CPU0 (no V) I was told hwcap still reported V !?
> My board just runs the big CPU1 core with V 1.0, so I have not verified this.

"At least" falsely reporting vector when there's none is a pretty simple
fix for the vendor (if they care). Having different versions of vector
on different CPUs would be much more annoying. And ye, I asked on IRC
and someone said it does report V as you say.

> > One thing that we did in the kernel was, when parsing a devicetree, to
> > ignore v in riscv,isa where the CPU vendor was T-Head and the archid was
> > 0x0. It wasn't meant to be a perfect detection of incorrect devicetrees,
> > but a simple best-effort attempt to handle vendor provided dtbs for
> > Vector 0.7 devices. Best-effort is fine in this case, cos ultimately it
> > isn't the kernel's problem if someone has incorrectly described their
> > hardware. Guo Ren claimed that no vector 1.0 devices have a 0x0 marchid,
> > but I do not know if there are vector 0.7 devices with a non-zero marchid.
> >
> 
> As we are talking about kernels without hwprobe, userspace can only
> get it if it's reported via proc. (?)
> (my k230 kernel for example does not)

Oh god, does that come with the crappy old "5.10.4" kernel from T-Head?
I was expecting that info to be available in /proc/cpuinfo, but IIRC it
was only added in 2022's 6.1. That "5.10.4" vendor kernel is a parasite.

> > I'd be very curious as to what the small cpu reports for marchid/mimpid,
> > to see if that rule would still hold true. If they happens to be non-zero,
> > mainline wouldn't even run on it, even ignoring vector, without either an
> > OpenSBI and devicetree replacement or kernel changes, so the courtesy
> > disabling of vector wouldn't even trigger. What it would mean, is a
> > similar approach to disable vector on 0.7 devices wouldn't suffice for
> > you.
> >
> > > needs to be handled, safe fetch vsetvli and vsetvli v1.0 check can fix
> > > those two examples.
> > > If we miss a board or a new board comes out we may lack proper check thus crash.
> >
> > With my only mainline matters blinkers on, that's another tick in the
> > "pressure on vendors to not ship old, heavily modified kernels" box!
> >
> > > In PRs:
> > > https://github.com/openjdk/jdk/pull/19472
> > > https://github.com/openjdk/jdk/pull/19679
> > >
> > > Options A was used.
> > >
> > > The question is if we can reconsider due to the benefits and use
> > > another option allowing V to automatically work?
> >
> > I think whether you reconsider isn't really a topic for lkml, it seems
> > more like a policy decision for your project as there's not a magic bullet
> > we can offer you that can be sure not to break if you intent supporting
> > "random" vendor kernels.
> 
> Yes, wrong list :) And I yes that's my opinion also.
> But as it will signicant make life easier for many, I'm willing to let it slide.
> 
> Added the correct list, but I'll keep lkml if anyone else wants to chime in.

Oh, I was almost not going to mention the list, glad I did if you
actually meant to send it elsewhere.

Cheers,
Conor.

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