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From: Arnd Bergmann <arnd@kernel.org>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: "grygorii.strashko@ti.com" <grygorii.strashko@ti.com>,
	"linux-snps-arc@lists.infradead.org"
	<linux-snps-arc@lists.infradead.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"ilias.apalodimas@linaro.org" <ilias.apalodimas@linaro.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	"linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"mgorman@suse.de" <mgorman@suse.de>,
	"brouer@redhat.com" <brouer@redhat.com>,
	"mcroce@linux.microsoft.com" <mcroce@linux.microsoft.com>,
	"mhocko@kernel.org" <mhocko@kernel.org>,
	"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
	"hch@lst.de" <hch@lst.de>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems
Date: Tue, 20 Apr 2021 23:20:19 +0200
Message-ID: <CAK8P3a3rzz1gfNLoGC8aZJiAC-tgZYD6P8pQsoEfgCAmQK=FAw@mail.gmail.com> (raw)
In-Reply-To: <8d0fce1c-be7c-1c9b-bf5c-0c531db496ac@synopsys.com>

On Tue, Apr 20, 2021 at 11:14 PM Vineet Gupta
<Vineet.Gupta1@synopsys.com> wrote:
> On 4/20/21 12:07 AM, Arnd Bergmann wrote:

> >
> > which means that half the 32-bit architectures do this. This may
> > cause more problems when arc and/or microblaze want to support
> > 64-bit kernels and compat mode in the future on their latest hardware,
> > as that means duplicating the x86 specific hacks we have for compat.
> >
> > What is alignof(u64) on 64-bit arc?
>
> $ echo 'int a = __alignof__(long long);' | arc64-linux-gnu-gcc -xc -
> -Wall -S -o - | grep -A1 a: | tail -n 1 | cut -f 3
> 8

Ok, good.

> Yeah ARCv2 alignment of 4 for 64-bit data was a bit of surprise finding
> for me as well. When 64-bit load/stores were initially targeted by the
> internal Metaware compiler (llvm based) they decided to keep alignment
> to 4 still (granted hardware allowed this) and then gcc guys decided to
> follow the same ABI. I only found this by accident :-)
>
> Can you point me to some specifics on the compat issue. For better of
> worse, arc64 does''t have a compat 32-bit mode, so everything is
> 64-on-64 or 32-on-32 (ARC32 flavor of ARCv3)

In that case, there should be no problem for you.

The main issue is with system calls and ioctls that contain a misaligned
struct member like

struct s {
       u32 a;
       u64 b;
};

Passing this structure by reference from a 32-bit user space application
to a 64-bit kernel with different alignment constraints means that the
kernel has to convert the structure layout. See
compat_ioctl_preallocate() in fs/ioctl.c for one such example.

       Arnd

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-16 23:07 [PATCH 0/2] Change struct page layout for page_pool Matthew Wilcox (Oracle)
2021-04-16 23:07 ` [PATCH 1/2] mm: Fix struct page layout on 32-bit systems Matthew Wilcox (Oracle)
2021-04-17  1:08   ` kernel test robot
2021-04-17  2:45   ` Matthew Wilcox
2021-04-17 18:32     ` Ilias Apalodimas
2021-04-17 20:22       ` Matthew Wilcox
2021-04-18 11:21         ` Ilias Apalodimas
2021-04-17 21:18     ` David Laight
2021-04-17 22:45       ` Matthew Wilcox
2021-04-20  2:48     ` Vineet Gupta
2021-04-20  3:10       ` Matthew Wilcox
2021-04-20  7:07         ` Arnd Bergmann
2021-04-20 21:14           ` Vineet Gupta
2021-04-20 21:20             ` Arnd Bergmann [this message]
2021-04-21  5:50               ` hch
2021-04-21  8:43               ` David Laight
2021-04-21  8:58                 ` Arnd Bergmann
2021-04-20  7:39     ` Geert Uytterhoeven
2021-04-20  8:39       ` David Laight
2021-04-20 11:32       ` Matthew Wilcox
2021-04-17  7:34   ` Jesper Dangaard Brouer
2021-04-20  7:21   ` Rasmus Villemoes
2021-04-16 23:07 ` [PATCH 2/2] mm: Indicate pfmemalloc pages in compound_head Matthew Wilcox (Oracle)
2021-04-17 21:13   ` David Laight
2021-04-17 21:28     ` Matthew Wilcox
2021-04-20  4:03 ` [PATCH 0/2] Change struct page layout for page_pool Michael Ellerman

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