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

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Date:   Mon, 23 Jan 2017 12:57:25 -0800
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     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, haokexin@...il.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 Mon, Jan 23, 2017 at 09:23:06AM -0800, Dave Hansen wrote:
> On 01/23/2017 08:55 AM, Yu-cheng Yu wrote:
> >> The best fix here would be not to paper over the issue in the copy
> >> function but find where it got clobbered, or where some initialization
> >> code failed to set it.
> > 
> > Someone else reported different issues from the same bug and a different
> > patch was just tested OK this morning.  I think that adding xfeatures bits
> > to xcomp_bv should have been done in fpstate_init().
> 
> Right.  So where did it get cleared out?

It is not set until a task triggers XSAVES.  We did not set it in fpstate_init()
because there is no valid data at the time.  The problem happens when Linux 
copies data to the XSAVES area, like we see here; the kernel is not expected
to change XSAVES format (xcomp_bv) but xcomp_bv is still blank (except bit 63). 

Because XSAVES format is not changed after boot time and xcomp_bv does not affect
INIT optimization, why don't we fix the problem in fpstate_init()?  

Yu-cheng