From: Peter Zijlstra <peterz@infradead.org>
To: Vineet Gupta <vineet.gupta1@synopsys.com>
Cc: David Laight <David.Laight@ACULAB.COM>,
Alexey Brodkin <alexey.brodkin@synopsys.com>,
"linux-snps-arc@lists.infradead.org"
<linux-snps-arc@lists.infradead.org>,
Arnd Bergmann <arnd.bergmann@linaro.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"stable@vger.kernel.org" <stable@vger.kernel.org>,
Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH] ARC: Explicitly set ARCH_SLAB_MINALIGN = 8
Date: Thu, 14 Feb 2019 11:31:40 +0100 [thread overview]
Message-ID: <20190214103140.GG32494@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <e6138aeb-9ce4-fbfa-4d6a-b76f189a3e47@synopsys.com>
On Wed, Feb 13, 2019 at 03:23:36PM -0800, Vineet Gupta wrote:
> On 2/13/19 4:56 AM, Peter Zijlstra wrote:
> >
> > Personally I think u64 and company should already force natural
> > alignment; but alas.
>
> But there is an ISA/ABI angle here too. e.g. On 32-bit ARC, LDD (load double) is
> allowed to take a 32-bit aligned address to load a register pair. Thus all u64
> need not be 64-bit aligned (unless attribute aligned 8 etc) hence the relaxation
> in ABI (alignment of long long is 4). You could certainly argue that we end up
> undoing some of it anyways by defining things like ARCH_KMALLOC_MINALIGN to 8, but
> still...
So what happens if the data is then split across two cachelines; will a
STD vs LDD still be single-copy-atomic? I don't _think_ we rely on that
for > sizeof(unsigned long), with the obvious exception of atomic64_t,
but yuck...
So even though it is allowed by the chip; does it really make sense to
use this?
next prev parent reply other threads:[~2019-02-14 10:31 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-08 10:55 [PATCH] ARC: Explicitly set ARCH_SLAB_MINALIGN = 8 Alexey Brodkin
2019-02-12 17:17 ` Vineet Gupta
2019-02-12 17:30 ` David Laight
2019-02-12 17:45 ` Vineet Gupta
2019-02-13 12:56 ` Peter Zijlstra
2019-02-13 14:13 ` David Laight
2019-02-13 23:23 ` Vineet Gupta
2019-02-14 8:50 ` Alexey Brodkin
2019-02-14 10:28 ` Peter Zijlstra
2019-02-15 1:34 ` Vineet Gupta
2019-02-18 8:53 ` Alexey Brodkin
2019-02-19 23:30 ` Vineet Gupta
2019-02-14 10:31 ` Peter Zijlstra [this message]
2019-02-14 10:44 ` Alexey Brodkin
2019-02-14 11:08 ` Peter Zijlstra
2019-02-14 12:05 ` Alexey Brodkin
2019-02-14 12:24 ` Peter Zijlstra
2019-02-14 14:14 ` David Laight
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