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

On 23 May 2013 22:57, Nicolas Pitre [off-list ref] wrote:

On Thu, 23 May 2013, Ard Biesheuvel wrote:

quoted

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.