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

[Ard Biesheuvel]

On 23 May 2013 22:57, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> On Thu, 23 May 2013, Ard Biesheuvel wrote:
>> 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.
>

So perhaps that is where we should draw the line: I am not proposing
supporting arbitrary VFP code in the kernel, just NEON (which does not
bounce and hence does not require the support code). Maybe I should
rename to kernel_neon_begin/kernel_neon_end to reflect that?

Regards,
Ard.