[PATCH] powerpc/kernel: improve FP and vector registers restoration

[Breno Leitao]

Hi Anton,

On Sat, Jun 03, 2017 at 08:04:11AM +1000, Anton Blanchard wrote:
> Hi Breno,
> 
> > Currently tsk->thread->load_vec and load_fp are not initialized
> > during a task creation, which set garbage to these variables
> > (non-zero value).
> 
> Nice catch! It seems like we should zero load_tm too though?

Yes, it seems we need to zero load_tm also, since it does not seem to be
zeroed anywhere else.

But I did some tests, and load_tm is always zero after start_thread()
is being called.

In fact, start_thread() is being called and pt_regs->load_tm is already
zero since the function start.

I also wrote a SystemTap script[1] to investigate it better, and I've
never seen a single load_tm != 0 in a my machine. I tested on both
POWER8 bare metal and KVM guests. (load_vec and load_fp happened to have
garbage all the time)

Any idea if this is just occasional event, or, if there is someone
zeroing it in an obscure code?

[1] https://github.com/leitao/htm_torture/blob/master/systemtap/load_tm_at_start_thread.stap