Re: Oops with tip/x86/fpu

Oleg Nesterov
2015-03-05T15:17:44Z

On 03/04, Borislav Petkov wrote:
>
> On Wed, Mar 04, 2015 at 08:06:51PM +0100, Oleg Nesterov wrote:
> > Thanks. I'll try to investigate tomorrow.
> >
> > Well, the kernel crashes because xrstor_state() is buggy, Quentin already
> > has a fix.
> >
> > But #GP should be explained...
>
> Could it be one of those conditions for which XRSTORS #GPs, like
>
> "If XRSTORS attempts to load MXCSR with an illegal value, a
> general-protection exception (#GP) occurs."
>
> for example? I'm looking at the SDM section for XRSTORS.
>
> I mean, math_state_restore() does init_fpu() and down that road we're
> allocating an FPU state ... but we did init_fpu() before too, in
> eager_fpu_init(). So what changed?

I _think_ that the difference is that eager_fpu_init()->xrstor_state()
was called before apply_alternatives(), so it used XRSTOR.

Note also that (before this commit) restore_fpu_checking() was almost
never called right after init_fpu(). If use_eager_fpu() == T.

After this commit the first xrstor_state() uses XRSTORS. And that is
how (I think) 'noxsaves' makes the difference.

So. I can be easily wrong, but so far I _think_ that this commit disclosed
another problem. And even if I am wrong and this commit is buggy, we need
to understand why ;)

I'll try to think about debugging patch, I can't reproduce this problem
on my machine...

Oleg.