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
