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

[Nicolin Chen <nicoleotsuka@gmail.com>]

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?

Thank you
Nicolin