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?
> /*
> 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.
