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Subject: Re: [PATCH v5 11/13] x86/xsaves: Add WARN_ON_FPU() when a disabled
 xstate component offset is requested for a compacted format
To: Yu-cheng Yu <yu-cheng.yu@intel.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>
References: <cover.1462816638.git.yu-cheng.yu@intel.com>
 <d0091f5d1cfdce59275c40f50c637f5ae451cc7b.1462816638.git.yu-cheng.yu@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <57311DC6.5000503@linux.intel.com>
Date: Mon, 9 May 2016 16:31:18 -0700
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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?