On Fri, Jun 19, 2020 at 11:01:44AM -0700, Andy Lutomirski wrote:
> On Fri, Jun 19, 2020 at 10:41 AM Borislav Petkov <[email protected]> wrote:
> >
> > From: Petteri Aimonen <[email protected]>
> >
> > Previously, kernel floating point code would run with the MXCSR control
> > register value last set by userland code by the thread that was active
> > on the CPU core just before kernel call. This could affect calculation
> > results if rounding mode was changed, or a crash if a FPU/SIMD exception
> > was unmasked.
> >
> > Restore MXCSR to the kernel's default value.
> >
> > [ bp: Carve out from a bigger patch by Petteri, add feature check, add
> > FNINIT call too (amluto). ]
>
> Acked-by: Andy Lutomirski <[email protected]>
>
> but:
>
> shouldn't kernel_fpu_begin() end with a barrier()?
the "fninit" thing is already asm volatile or do you want the explicit
memory clobber of barrier?
If so, why?
The LDMXCSR and FNINIT have effect only on hardware state...
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
