[PATCH resend 1/2] arm64: defer reloading a task's FPSIMD state to userland resume

[Will Deacon]

Hi Ard,

On Fri, Jan 31, 2014 at 10:13:15AM +0000, Ard Biesheuvel wrote:
> If a task gets scheduled out and back in again and nothing has touched
> its FPSIMD state in the mean time, there is really no reason to reload
> it from memory. Similarly, repeated calls to kernel_neon_begin() and
> kernel_neon_end() will preserve and restore the FPSIMD state every time.
> 
> This patch defers the FPSIMD state restore to the last possible moment,
> i.e., right before the task re-enters userland. If a task does not enter
> userland at all (for any reason), the existing FPSIMD state is preserved
> and may be reused by the owning task if it gets scheduled in again on the
> same CPU.

The one situation I'm unsure of here is how you deal with the saved fpsimd
state potentially being updated by a signal handler or a debugger. In this
case, we probably need to set _TIF_FOREIGN_FPSTATE to force a reload, or are
you handling this some other way?

Will