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SubjectRe: [PATCH v3 9/9] x86/xsaves: Re-enable XSAVES
On Tue, Mar 01, 2016 at 03:56:12PM -0800, Dave Hansen wrote:
> On 02/29/2016 09:42 AM, Yu-cheng Yu wrote:
> > /*
> > - * 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. )
> > + * Most recent CPUs supporting XSAVES can run 64-bit mode.
> > + * Enable XSAVES for 64-bit.
> > */
> > - setup_clear_cpu_cap(X86_FEATURE_XSAVES);
> > + if (!config_enabled(CONFIG_X86_64))
> > + setup_clear_cpu_cap(X86_FEATURE_XSAVES);
> > }
>
> I think we need a much better explanation of this for posterity. Why
> are we not supporting this now, and what would someone have to do in the
> future in order to enable it?
>
If anyone is using this newer feature, then that user is most likely using
a 64-bit capable processor and a 64-bit kernel. The intention here is to
take out the complexity and any potential of error. If the user removes
the restriction and builds a private kernel, it should work but we have
not checked all possible combinations. I will put these in the comments.

> > /*
> > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> > index 2e80d6f..cb2a484 100644
> > --- a/arch/x86/kernel/fpu/xstate.c
> > +++ b/arch/x86/kernel/fpu/xstate.c
> > @@ -204,6 +204,14 @@ void fpu__init_cpu_xstate(void)
> > if (!cpu_has_xsave || !xfeatures_mask)
> > return;
> >
> > + /*
> > + * Make it clear that XSAVES supervisor states are not yet
> > + * implemented should anyone expect it to work by changing
> > + * bits in XFEATURE_MASK_* macros and XCR0.
> > + */
> > + WARN_ONCE((xfeatures_mask & XFEATURE_MASK_SUPERVISOR),
> > + "x86/fpu: XSAVES supervisor states are not yet implemented.\n");
> > +
> > cr4_set_bits(X86_CR4_OSXSAVE);
> > xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask);
> > }
>
> Let's also do a:
>
> xfeatures_mask &= ~XFEATURE_MASK_SUPERVISOR;
>
> Otherwise, we have a broken system at the moment.
>
Currently, if anyone sets any supervisor state in xfeatures_mask, the
kernel prints out the warning then goes into a protection fault.
That is a very strong indication to the user. Do we want to mute it?

Yu-cheng

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