Date:   Sun, 26 Feb 2023 21:56:44 +0100
From:   Heiko Carstens <hca@linux.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Thomas Richter <tmricht@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] s390/rwonce: add READ_ONCE_ALIGNED_128() macro
Message-ID: <Y/vHjFRIJTTKVcFq@osiris>
References: <20230224100237.3247871-1-hca@linux.ibm.com>
 <20230224100237.3247871-2-hca@linux.ibm.com>
 <Y/o8cmyc7IW+28S2@hirez.programming.kicks-ass.net>
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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.