Re: [PATCH 00/19] x86, mpx updates for 4.2 (take 7)

[Ingo Molnar <mingo@kernel.org>]

* Dave Hansen <dave@sr71.net> wrote:

> On 05/20/2015 03:05 AM, Ingo Molnar wrote:
> >> > 
> >> > This sees breakage unless either booted with 'noxsaves'
> >> > or if it has Fenghua's set from here applied:
> >> > 
> >> > 	http://lkml.kernel.org/r/1429678319-61356-1-git-send-email-fenghua.yu@intel.com
> >> > 
> >> > This set is also available against 4.1-rc3 in git:
> >> > 
> >> >   git://git.kernel.org/pub/scm/linux/kernel/git/daveh/x86-mpx.git mpx-v22
>
> > Yeah, so as a first step, could you please test that the patch below solves 
> > the crashes as well, without having to specify 'noxsaves' on the boot line?
> 
> Yes, that does seem to make it happy in lieu of Fenghua's patches.

Great, thanks for testing it! I'll send it to Linus as part of x86/urgent.

> > +	/*
> > +	 * Quirk: we don't yet handle the XSAVES* instructions
> > +	 * correctly, as we don't correctly convert between
> > +	 * standard and compacted format when interfacing
> > +	 * with user-space - so disable it for now.
> > +	 *
> > +	 * The difference is small: with recent CPUs the
> > +	 * compacted format is only marginally smaller than
> > +	 * the standard FPU state format.
> > +	 *
> > +	 * ( This is easy to backport while we are fixing
> > +	 *   XSAVES* support. )
> > +	 */
> > +	setup_clear_cpu_cap(X86_FEATURE_XSAVES);
> >  }
> 
> FWIW, I think it would be prudent to also clear X86_FEATURE_XSAVEC.

Ok, we can do that as well - but it's technically not needed to keep the kernel 
from generating compact format FPU saves.

I'd also hope we can restore compact format handling soon-ish.

> All of the issues I am aware of are related to the compact format, not 'xsaves' 
> itself (although 'xsaves' does *use* the compact format of course).
> 
> The XSAVEC bit is the one that technically indicates the compact format support, 
> although I don't think there is any actual use of it in the kernel at present.

Yeah.

Thanks,

	Ingo