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Subject: Re: [tip:x86/urgent] x86/fpu: Set the xcomp_bv when we fake up a
 XSAVES area
To: Kevin Hao <haokexin@gmail.com>
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From: Dave Hansen <dave.hansen@linux.intel.com>
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Date: Mon, 23 Jan 2017 18:01:10 -0800
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On 01/23/2017 05:50 PM, Kevin Hao wrote:
> According to the ISA manual, XSAVES also set the XCOMP_BV[62:0]. My code only
> try to be compatible with what the cpu does when excuting XSAVES. The following
> is quoted from 325462-sdm-vol-1-2abcd-3abcd.pdf.
>   The XSAVES instructions sets bit 63 of the XCOMP_BV field of the XSAVE header while writing RFBM[62:0] to
>   XCOMP_BV[62:0]. The XSAVES instruction does not write any part of the XSAVE header other than the XSTATE_BV
>   and XCOMP_BV fields.

What purpose does it serve to make copyin_to_xsaves() set that bit,
other than helping to hide bugs?