linux-kernel - Re: [PATCH v3 9/9] x86/xsaves: Re-enable XSAVES

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Date:	Tue, 1 Mar 2016 15:56:12 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Cc:	Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v3 9/9] x86/xsaves: Re-enable XSAVES

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.