On Mon, Jun 22, 2020 at 10:09 AM Borislav Petkov <b...@alien8.de> wrote: > > On Fri, Jun 19, 2020 at 11:01:44AM -0700, Andy Lutomirski wrote: > > On Fri, Jun 19, 2020 at 10:41 AM Borislav Petkov <b...@alien8.de> wrote: > > > > > > From: Petteri Aimonen <j...@git.mail.kapsi.fi> > > > > > > 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 <l...@kernel.org> > > > > 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... >
Suppose you do: double x = 1.0; kernel_fpu_begin(); x += 2.0; We want to make sure that GCC puts things in the right order. I suppose that even a memory clobber is insufficient here, though.
