linux-kernel - Re: [PATCH v5 11/13] x86/xsaves: Add WARN_ON_FPU() when a disabled xstate component offset is requested for a compacted format

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Date:	Mon, 9 May 2016 16:31:18 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
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 v5 11/13] x86/xsaves: Add WARN_ON_FPU() when a disabled
 xstate component offset is requested for a compacted format

On 05/09/2016 01:46 PM, Yu-cheng Yu wrote:
> Add a warning in case a disabled (not existing) xstate component offset
> is requested.
...
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 350814c..2e6dbfe 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -756,6 +756,7 @@ void *__raw_xsave_addr(struct xregs_state *xsave, int xstate_feature_mask)
>  {
>  	int feature_nr = fls64(xstate_feature_mask) - 1;
>  
> +	WARN_ON_FPU(using_compacted_format() && !xfeature_enabled(feature_nr));
>  	return (void *)xsave + xstate_comp_offsets[feature_nr];
>  }

Why the using_compacted_format()?  Shouldn't this be an error, regardless.

Also, what is xstate_comp_offsets[feature_nr] in this case?  Isn't it
-1?  Should we be returning NULL along with WARN()ing?