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

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Date:   Mon, 23 Jan 2017 08:55:30 -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 07:36:20AM -0800, Dave Hansen wrote:
> The kernel xsave buffer should *ALWAYS* have the
> XCOMP_BV_COMPACTED_FORMAT bit set.  It should have been set before the
> copyin and it should be set when it's finished.
> 
> 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().

Also, in copy_init_fpstate_to_fpregs(), we do:

	copy_kernel_to_xregs(&init_fpstate.xsave, -1).

That (-1) could mean (0) because the parameters are declared as:

	copy_kernel_to_xregs(struct xregs_state *, u64)

Yu-cheng