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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/03/2024 3:33 pm, 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'm not sure it will help for bisectability in this case.  But it might
simplify some of the other rearrangements.


>> 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.
>
>> 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.

The argument being made is that it seems highly unlikely for there to be
non-Zbb systems running Xen, and furthermore, if this turns out not to
be true, it is reasonable to offload the effort of making it work to
whomever has hardware looking like that.

i.e. it's fine to require Zbb at this point.

This doesn't prevent someone else doing the work to alter this in the
future, for what appears to be an absurd configuration that is unlikely
to exist in reality.

~Andrew