Re: [PATCH v5 12/23] xen/riscv: introduce io.h

[Jan Beulich <jbeulich@suse.com>]

On 07.03.2024 21:49, Oleksii wrote:
> On Thu, 2024-03-07 at 18:14 +0100, Jan Beulich wrote:
>> For plain writes it should at least be "=Qo" then, yes.
> Constraints Q is a machine specific constraint, and I am not sure that
> it makes sense to use "=o" only and probably it is a reason why it is
> enough only "r". Does it make sense?

Especially the 'only "r"' part doesn't, no. I also don't see why "=o"
would make no sense - that's fundamentally no different than "=m".
Unless the immediates used in the ultimate insns are large enough to
cover the full range of possible values, my take is that "o" is never
appropriate to use. With one exception in a case like we have here:
If the operand isn't used in the actual instruction(s), then that's
fine, as the specific value of the adjusted immediate wouldn't matter
at all.

As to "Q" - that's an Arm constraint anyway, not a RISC-V one? If so,
I'm not sure why we're discussing it here. In any event, I'd be curious
to understand in how far the combination "Qo" makes sense.

>>  To me making those
>> input operands on Arm can't have been quite right.
> I  don't understand why they both are input, logically it looks like an
> address should be an output.

How would an address be an output, when that's needed to be calculated
for the memory access to take place? It would be an output (and
presumably a dummy/fake one) only if the address calculation was done
in the asm() itself (and there I don't mean the operands, but the
actual assembly instruction(s)).

Jan