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

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Date:	Sat, 15 Jan 2011 15:43:19 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Colin Cross <ccross@...roid.com>
Subject: Re: [PATCH] ARM: vfp: Fix up exception location in Thumb mode

On Sat, Jan 15, 2011 at 03:38:16PM +0000, Catalin Marinas wrote:
> On 14 January 2011 18:47, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > diff -u b/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> > --- b/arch/arm/kernel/entry-armv.S
> > +++ b/arch/arm/kernel/entry-armv.S
> > @@ -499,10 +499,11 @@
> >        blo     __und_usr_unknown
> >  3:     ldrht   r0, [r4]
> >        add     r2, r2, #2                      @ r2 is PC + 2, make it PC + 4
> > -       orr     r0, r0, r5, lsl #16
> > +       str     r2, [sp, #S_PC]                 @ it's a 2x16bit instr, update
> > +       orr     r0, r0, r5, lsl #16             @  regs->ARM_pc
> >        @
> >        @ r0 = the two 16-bit Thumb instructions which caused the exception
> > -       @ r2 = PC value for the following Thumb instruction (:= regs->ARM_pc+2)
> > +       @ r2 = PC value for the following Thumb instruction (:= regs->ARM_pc)
> >        @ r4 = PC value for the first 16-bit Thumb instruction
> >        @
> >  #else
> 
> Do we need to modify the VFP entry code to avoit the store to ARM_pc?

The one after the sub #4 instruction?

That's answered by the comments... "retry the instruction" and that
r2 = regs->ARM_pc in every case, and both r2 and regs->ARM_pc point
at the _following_ instruction...

I do hope this isn't a case that _more_ comments are making this more
confusing (which seems to be the way with documentation - the more
words you use, the more questions people have).  Maybe we should get
rid of all the comments instead?
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