Re: [RISCV] [PATCH 0/7] xen/bitops: Reduce the mess, starting with ffs()

[Oleksii <oleksii.kurochko@gmail.com>]

On Thu, 2024-03-14 at 16:33 +0100, Jan Beulich wrote:
> On 14.03.2024 15:45, Andrew Cooper wrote:
> > On 13/03/2024 5:27 pm, Andrew Cooper wrote:
> > > Start cleaning it up with ffs() and friends.  Across the board,
> > > this adds:
> > > 
> > >  * Functioning bitops without arch-specific asm
> > 
> > It turns out that RISC-V doesn't have a CLZ instruction in the base
> > ISA.  As a consequence, __builtin_ffs() emits a library call to
> > ffs() on
> > GCC, or a de Bruijn sequence on Clang.
> > 
> > The optional Zbb extension adds a CLZ instruction, after which
> > __builtin_ffs() emits a very simple sequence.
> > 
> > This leaves us with several options.
> > 
> > 1) Put generic_ffs() back in, although if we do this then it's
> > going to
> > be out-of-line in lib/ where it can be mostly ignored.
> > 
> > 2) Require Zbb for Xen.
> > 
> > 3) Alternative it up with Zbb or generic_ffs().
> > 
> > 
> > I've got half a mind to do 1) irrespective.  It's mostly just
> > shuffling
> > logic out of bitops.h into lib/.
> 
> Yes. Might also help with the bi-sectability issue you faced.
> 
> > I also think we should do option 2 for RISCV.  Given the
> > instruction
> > groups that H does mandate, it's unrealistic to expect that such a
> > chip
> > wouldn't support Zbb/etc.
> 
> I'm not so sure here.
If to look at available specs of CPUs with H, then, for example, SiFive
P600 series family doesn't support Zbb extenstion:
https://sifive.cdn.prismic.io/sifive/7be0420e-dac1-4558-85bc-50c7a10787e7_p600_datasheet.pdf

But I asked a team who are producing CPU with H support and they have
Zbb extenstion.

> 
> > Also, getting full alternatives working is yet-more work that's not
> > trivial at this point in RISCV's development.  I think it is
> > entirely
> > reasonable to avoid this work for now, and make it a problem for
> > anyone
> > who has an H-capable Zbb-incapable system.  (with a strong
> > implication
> > that this is work that probably never needs to be done.)
> 
> That's definitely for later.
Considering that we are mainly using QEMU and it provides Zbb extension
we can just update -march, and that a real h/w where I can ask to test
a code also support this extenstion I will update riscv/booting.txt and
update -march.

~ Oleksii