Re: [PATCH 1/2] s390/rwonce: add READ_ONCE_ALIGNED_128() macro

[Heiko Carstens]

On Sat, Feb 25, 2023 at 05:50:58PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 24, 2023 at 11:02:36AM +0100, Heiko Carstens wrote:
> > Add an s390 specific READ_ONCE_ALIGNED_128() helper, which can be used for
> > fast block concurrent (atomic) 128-bit accesses.
> > 
> > The used lpq instruction requires 128-bit alignment. This is also the
> > reason why the compiler doesn't emit this instruction if __READ_ONCE() is
> > used for 128-bit accesses.
> 
> Does your u128 not have natural alignment? Does it help if you force
> align the u128 type?

s390 seems to be the only architecture which has a 64 bit alignment for
__uint128_t. But making it explicitly naturally aligned doesn't help.
I guess that's because the lpq instruction requires an even-odd register
pair where it reads to, while the now used lmg instruction can use any
register pair; but lmg doesn't come with atomic semantics.