linux-kernel - Re: [PATCH] ARM: vfp: Fix up exception location in Thumb mode

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Message-ID: <1295014231.7901.41.camel@e102109-lin.cambridge.arm.com>
Date:	Fri, 14 Jan 2011 14:10:31 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Colin Cross <ccross@...roid.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: vfp: Fix up exception location in Thumb mode

On Fri, 2011-01-14 at 12:02 +0000, Russell King - ARM Linux wrote:
> On Fri, Jan 14, 2011 at 11:43:04AM +0000, Catalin Marinas wrote:
> > >        pr_debug("VFP: bounce: trigger %08x fpexc %08x\n", trigger, fpexc);
> > >
> > >        /*
> > > +        * If the exception occured in thumb mode, pc is exception location + 2,
> > > +        * the middle of the 32-bit VFP instruction.  Add 2 to get exception
> > > +        * location + 4, the same we get in ARM mode.
> > > +        */
> > > +#ifdef CONFIG_ARM_THUMB
> > > +       if (regs->ARM_cpsr & PSR_T_BIT)
> > > +               regs->ARM_pc += 2;
> > > +#endif
> >
> > You can use "if (thumb_mode(regs))" and avoid the #ifdef entirely.
> 
> I don't think this is correct.  On entry to the undefined instruction
> handler, we get the uncorrected PC value, so PC points to the
> instruction after the faulting instruction.
> 
> If it was an ARM instruction, that is located at PC-4.  If it was a
> Thumb instruction, it is located at PC-2.  This PC value is passed
> unmodified to the VFP entry code, and the passed r2 reflect the
> value in regs->ARM_pc.

The entry-armv.S code adds 2 to the r2 register in case of a 32-bit
Thumb instruction, so it is no longer the same as the ARM_pc.

Since the VFP instructions in Thumb mode are always 32-bit, Colin's
patch made sense to me.

> I think that the undefined instruction handling needs reworking for
> Thumb entirely as we could be dealing with a 16-bit or 32-bit thumb
> instruction, and we have no way of knowing without repeatedly
> decoding that instruction.

We already handle the r2 for in __und_usr. We don't deal with ARM_pc but
we could either do it in __und_usr or let the code handling the undef
fix it up.

-- 
Catalin


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