[PATCH 2/2] ARM: move VFP init to an earlier boot stage

[Nicolas Pitre]

On Thu, 23 May 2013, Ard Biesheuvel wrote:

> On 23 May 2013 17:23, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> > On Thu, 23 May 2013, Russell King - ARM Linux wrote:
> >
> >> Secondly, it makes it too easy for people to start thinking that FP is
> >> safe in the kernel.  It isn't - this will go horribly wrong if the VFP
> >> hardware bounces an instruction to the support code.
> >
> > Hmmm. good point.  We probably should set a flag in kernel_vfp_begin()
> > and BUG() if the support code is invoked while that flag is set.
> >
> 
> Currently, the lazy restore is armed even in kernel mode, so it pulls
> in the userland context if you issue NEON/VFP instructions in kernel
> mode with the FP unit off. This is probably a side effect of the VFP
> detection code relying on this code path as well.
> As kernel_vfp_begin() disarms the lazy restore and takes care of
> preserving the userland context itself, entering __und_svc because of
> NEON/VFP instructions should only be allowed in this particular case,
> i.e., VFP detection and all other instances should BUG() imho
> (regardless of if we end up doing NEON in the kernel or not). Doing it
> that way, I don't think there is a reason for keeping additional
> flags.

The concern is more about VFP instructions that could trigger an 
exception where support code is invoked to emulate that instruction.


Nicolas