linux-kernel - Re: [PATCH] arm64: mm: support instruction SETEND

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Message-ID: <20150107142513.GB2199@e104818-lin.cambridge.arm.com>
Date:	Wed, 7 Jan 2015 14:25:14 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Suzuki Poulose <Suzuki.Poulose@....com>,
	Leo Yan <leo.yan@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Xiaolong Ye <yexl@...vell.com>
Subject: Re: [PATCH] arm64: mm: support instruction SETEND

On Wed, Jan 07, 2015 at 10:25:43AM +0000, Will Deacon wrote:
> On Wed, Jan 07, 2015 at 10:10:34AM +0000, Suzuki K. Poulose wrote:
> > On 07/01/15 05:52, Leo Yan wrote:
> > > Currently kernel has set the bit SCTLR_EL1.SED, so the SETEND
> > > instruction will be treated as UNALLOCATED; this error can be
> > > reproduced when ARMv8 cpu runs with EL1/aarch64 and EL0/aarch32
> > > mode, finally kernel will trap the exception if the userspace
> > > libs use SETEND instruction.
> > >
> > > So this patch clears bit SCTLR_EL1.SED to support SETEND instruction.
> > >
> > The best way to do this, is via the instruction emulation framework 
> > added by Punit, which handles the armv8 deprecated/obsoleted 
> > instructions. This is now queued for 3.19.
> > I have a patchset which adds the 'SETEND' emulation support to the 
> > framework. This will enable better handling of the feature, including 
> > finding out the users of the deprecated instruction (when we switch to 
> > the emulation mode).
> > 
> > Btw, there is one open question that I am seeking answer for.
> > 
> > What should be the endianness of the signal handlers ? Should we leave 
> > it to the application ? Or restore the 'default' endianness for the 
> > signal handler ?
> 
> I think we should restore the default endianness, otherwise you're
> essentially forcing signal handlers to do a setend as their first
> instruction to get into a consistent state. That also matches the endianness
> of the sigframe that we push onto the stack, right?

It looks like this is what setup_return() already does on arm32
(switching to the default endianness).

Now that we plan to allow SETEND in AArch32 applications, we need
something similar in the arm64 compat_setup_return().

-- 
Catalin
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