Re: [PATCH RFC v1] arm64: Handle traps from accessing CNTVCT/CNTFRQ for CONFIG_COMPAT

[Robin Murphy <robin.murphy@arm.com>]

On Wed, 17 Jan 2018 12:41:56 -0800
Nicolin Chen <nicoleotsuka@gmail.com> wrote:

> On Wed, Jan 17, 2018 at 09:03:48AM +0000, Marc Zyngier wrote:
> 
> > > So ignoring a condition for a Thumb instruction may cause its IT
> > > scope shifting. For ARM mode, the only penalty could be two Rts
> > > getting written -- which shouldn't corrupt userspace execution.
> > > 
> > > Please correct me if I am wrong or not thorough.  
> > 
> > Consider the following:
> > 	
> > 	mov	r0, #0
> > 	mov	r1, #0
> > 	cmp	r1, #3
> > 	mrrceq	r0, r1, cntvct // simplified version
> > 
> > Oh look, you've corrupted r0 and r1, which should never have be
> > changed. Whatever uses the content r0 and r1 after the mrrc will
> > misbehave. How is that an acceptable behaviour? How do you expect
> > userspace to cope with such a brain damage?
> > 
> > If you intend to emulate the CPU, you must emulate it fully, to the
> > letter of the architecture. No ifs, no buts.  
> 
> Thanks for the explain. I see the point here.
> 
> I saw your version for arm64 compat doesn't check if (rt != 31)
> as MRS handler does. Is there any reason for that?

Um, perhaps because it's *compat*? How do you envision an AArch32
MR{R}C instruction targeting r31, exactly? ;)

Robin.