linux-kernel - Re: [PATCH 05/19] arm64: rename COMPAT to AARCH32

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Message-ID: <20160811145000.GB18366@e104818-lin.cambridge.arm.com>
Date:	Thu, 11 Aug 2016 15:50:00 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"Zhangjian (Bamvor)" <bamvor.zhangjian@...wei.com>,
	Yury Norov <ynorov@...iumnetworks.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-arch@...r.kernel.org,
	libc-alpha@...rceware.org, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, pinskia@...il.com, broonie@...nel.org,
	joseph@...esourcery.com, christoph.muellner@...obroma-systems.com,
	szabolcs.nagy@....com, klimov.linux@...il.com,
	Nathan_Lynch@...tor.com, agraf@...e.de,
	Prasun.Kapoor@...iumnetworks.com, kilobyte@...band.pl,
	geert@...ux-m68k.org, philipp.tomsich@...obroma-systems.com,
	manuel.montezelo@...il.com, linyongting@...wei.com,
	maxim.kuvyrkov@...aro.org, davem@...emloft.net,
	Andrew Pinski <apinski@...ium.com>,
	Andrew Pinski <Andrew.Pinski@...iumnetworks.com>,
	Bamvor Jian Zhang <bamvor.zhangjian@...aro.org>,
	Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH 05/19] arm64: rename COMPAT to AARCH32_EL0 in Kconfig

On Thu, Aug 11, 2016 at 10:53:01AM +0200, Arnd Bergmann wrote:
> On Thursday, August 11, 2016 3:35:01 PM CEST Zhangjian (Bamvor) wrote:
> > On 2016/6/18 7:54, Yury Norov wrote:
> > > From: Andrew Pinski <apinski@...ium.com>
> > >
> > > In this patchset  ILP32 ABI support is added. Additionally to AARCH32,
> > > which is binary-compatible with ARM, ILP32 is (mostly) ABI-compatible.
> > >
> > >  From now, AARCH32_EL0 (former COMPAT) config option means the support of
> > > AARCH32 userspace, ARM64_ILP32 - support of ILP32 ABI (see next patches),
> > > and COMPAT indicates that one of them, or both, is enabled.
> > >
> > > Where needed, CONFIG_COMPAT is changed over to use CONFIG_AARCH32_EL0 instead
> > >
> > > Reviewed-by: David Daney <ddaney@...iumnetworks.com>
> > > Signed-off-by: Andrew Pinski <Andrew.Pinski@...iumnetworks.com>
> > > Signed-off-by: Philipp Tomsich <philipp.tomsich@...obroma-systems.com>
> > > Signed-off-by: Christoph Muellner <christoph.muellner@...obroma-systems.com>
> > > Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@...aro.org>
> > > Signed-off-by: Yury Norov <ynorov@...iumnetworks.com>
> > ...
> > > diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
> > > index c173d32..af200a8 100644
> > > --- a/arch/arm64/kernel/cpuinfo.c
> > > +++ b/arch/arm64/kernel/cpuinfo.c
> > > @@ -134,15 +134,17 @@ static int c_show(struct seq_file *m, void *v)
> > >   		 */
> > >   		seq_puts(m, "Features\t:");
> > >   		if (compat) {
> > > -#ifdef CONFIG_COMPAT
> > > -			for (j = 0; compat_hwcap_str[j]; j++)
> > > -				if (compat_elf_hwcap & (1 << j))
> > > -					seq_printf(m, " %s", compat_hwcap_str[j]);
> > > -
> > > -			for (j = 0; compat_hwcap2_str[j]; j++)
> > > -				if (compat_elf_hwcap2 & (1 << j))
> > > -					seq_printf(m, " %s", compat_hwcap2_str[j]);
> > > -#endif /* CONFIG_COMPAT */
> > > +#ifdef CONFIG_AARCH32_EL0
> > I saw that compat_hwcap_str and compat_hwcap2_str is defined when
> > "CONFIG_COMPAT" is true. Why we only change it to CONFIG_AARCH32_EL0
> > in c show()?
> > > +			if (personality(current->personality) == PER_LINUX32) {
> > And "compat" is "personality(current->personality) == PER_LINUX32;",
> > it seems that there is no need to add this twice.
> 
> I think it would be best to remove the #ifdef here completely,
> the PER_LINUX32 concept is not strictly tied to the emulation
> of ARM binaries, it literally just changes the output of
> /proc/cpuinfo and 'uname',

It's not strictly related to ARM binaries, however it is related to
AArch32 CPU features being supported and detected by the kernel.
Currently, with CONFIG_COMPAT disabled, we won't have access to a
(meaningful) compat_elf_hwcap.

> and you can have ARM binaries with
> PER_LINUX (using the arm64 uname) just like you can have
> arm64 binaries running with PER_LINUX32.

I was actually looking to enforce the 32-bit binaries to only see
PER_LINUX32, though with a risk of breaking the ABI. OTOH, people are
abusing this and write 32-bit apps relying on the 64-bit /proc/cpuinfo:

http://lkml.kernel.org/g/1464706504-25224-3-git-send-email-catalin.marinas@arm.com

(you were summoned on that discussion couple of times ;))

-- 
Catalin