linux-kernel - Re: [tip:x86/urgent] x86/fpu: Set the xcomp

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Message-ID: <2b638f89-70b4-7c0b-a271-a430157c5bfd@linux.intel.com>
Date:   Mon, 23 Jan 2017 18:38:42 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Kevin Hao <haokexin@...il.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>, fenghua.yu@...el.com,
        dvlasenk@...hat.com, peterz@...radead.org, oleg@...hat.com,
        mingo@...nel.org, linux-kernel@...r.kernel.org, brgerst@...il.com,
        luto@...nel.org, bp@...en8.de, jpoimboe@...hat.com, hpa@...or.com,
        quentin.casasnovas@...cle.com, tglx@...utronix.de,
        torvalds@...ux-foundation.org, riel@...hat.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/fpu: Set the xcomp_bv when we fake up a
 XSAVES area

On 01/23/2017 06:09 PM, Kevin Hao wrote:
> On Mon, Jan 23, 2017 at 06:01:10PM -0800, Dave Hansen wrote:
>> 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,
> We try to fake up a memory area which is supposed to be composed by XSAVES
> instruction. My code is just trying to do what the XSAVES do.

No. copyin_to_xsaves() copies data into an *existing* XSAVES-formatted
buffer.  If you want to change what it does, fine, but that's not what
it does or tries to do today.

>> other than helping to hide bugs?
> Why do you think it hide the bug? In contrast, I think my patch fixes what the
> bug really is. The memory area we fake up is bug, we should fix it there.

Yu-cheng found the bug.  That bug will probably manifest in other code
paths than copyin_to_xsaves().  If we did your patch, it would hide the
bug in those other code paths.