linux-kernel - RE: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use user_xstate

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Date:	Wed, 22 Apr 2015 19:05:21 +0000
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	"Hansen, Dave" <dave.hansen@...el.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>,
	"Williamson, Glenn P" <glenn.p.williamson@...el.com>
CC:	linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: RE: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use
 user_xstate_size for xstate size in signal context

> From: Hansen, Dave
> Sent: Wednesday, April 22, 2015 11:45 AM
> To: Yu, Fenghua; H. Peter Anvin; Ingo Molnar; Thomas Gleixner; Mallick, Asit
> K; Williamson, Glenn P
> Cc: linux-kernel; x86
> Subject: Re: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use
> user_xstate_size for xstate size in signal context
> 
> On 04/21/2015 09:51 PM, Fenghua Yu wrote:
> > +	/*
> > +	 * Copy rest of xstates in compact format to user.
> > +	 */
> > +	for (i = 2; i < xstate_features; i++) {
> > +		if (test_bit(i, (unsigned long *)&pcntxt_mask)) {
> > +			int user_offset, kernel_offset;
> > +			int size;
> > +
> > +			user_offset = xstate_offsets[i];
> > +			kernel_offset = xstate_comp_offsets[i];
> > +			size = xstate_sizes[i];
> > +
> > +			if (__copy_to_user(buf_fx + user_offset,
> > +				     xsave + kernel_offset, size))
> > +				return -1;
> > +		}
> > +	}
> 
> Is this checking the right bitmap?
> 
> The 'xsaves' documentation says: "If RFBM[i] = 1, XSTATE_BV[i] is set to the
> value of XINUSE[i]".  Where "XINUSE denotes the state-component bitmap
> corresponding to the init optimization".
> 
> So shouldn't this be checking xsave->xsave_hdr.xstate_bv instead of
> pcntxt_mask?  The will be equal unless the "init optimization" is in play.

Xsave->xsave_hdr.xstate_bv is equal to pcntxt_mask (see setup_init_fpu_buf()).

And here XINUSE and RFBM are irrelevant because they are used in init or modified
optimization during xsave*/xrstore*. We only copy out the xstates from kernel
to user and XINUSE and RFBM are not in scope here.

So I would think pcntxt_mask is good to be used here.

Thanks.

-Fenghua
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