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
Date: Sun, 26 Feb 2023 21:56:44 +0100 [thread overview]
Message-ID: <Y/vHjFRIJTTKVcFq@osiris> (raw)
In-Reply-To: <Y/o8cmyc7IW+28S2@hirez.programming.kicks-ass.net>
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.
next prev parent reply other threads:[~2023-02-26 20:57 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-24 10:02 [PATCH 0/2] s390: don't use 128-bit cmpxchg for READ_ONCE() purposes Heiko Carstens
2023-02-24 10:02 ` [PATCH 1/2] s390/rwonce: add READ_ONCE_ALIGNED_128() macro Heiko Carstens
2023-02-25 16:50 ` Peter Zijlstra
2023-02-26 20:56 ` Heiko Carstens [this message]
2023-02-27 11:51 ` Peter Zijlstra
2023-02-24 10:02 ` [PATCH 2/2] s390/cpum_sf: use READ_ONCE_ALIGNED_128() instead of 128-bit cmpxchg Heiko Carstens
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