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

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Date:   Tue, 24 Jan 2017 09:50:12 +0800
From:   Kevin Hao <haokexin@...il.com>
To:     Dave Hansen <dave.hansen@...ux.intel.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 Mon, Jan 23, 2017 at 04:53:25PM -0800, Dave Hansen wrote:
> >> The fix I am proposing is...
> >>
> >> 	state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
> >> 				       xfeatures_mask;
> > 
> > Actually I thought about this change before I made this patch, but I don't this
> > is the right fix. It is always error prone to init the xcomp_bv to all the
> > supported feature. In case like copyin_to_xsaves(), it is possible that the
> > features which should be set in xcomp_bv do not equal to all the supported
> > features. Please see the following codes in copyin_to_xsaves():
> > 	/*
> > 	 * The state that came in from userspace was user-state only.
> > 	 * Mask all the user states out of 'xfeatures':
> > 	 */
> > 	xsave->header.xfeatures &= XFEATURE_MASK_SUPERVISOR;
> > 
> > 	/*
> > 	 * Add back in the features that came in from userspace:
> > 	 */
> > 	xsave->header.xfeatures |= xfeatures;
> 
> Hi Kevin,
> 
> I think you may be confusing 'xfeatures' with 'xcomp_bv'.  xfeatures
> tells you what features are present in the buffer while xcomp_bv tells
> you what *format* the buffer is in.

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.

Thanks,
Kevin

> 
> Userspace never dictates the *format* of the kernel buffer, only the
> contents of each state.  So, it makes sense that the copyin code would
> not (and should not) modify xcomp_bv.
> 
> We ensure that xcomp_bv has all supported states set all the time, or
> we're *supposed* to.

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