linux-kernel - Re: [PATCH 07/10] x86/xsaves: Fix init_fpstate.header.xcomp

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Message-ID: <56CB67C6.3050707@linux.intel.com>
Date:	Mon, 22 Feb 2016 11:55:50 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Cc:	Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH 07/10] x86/xsaves: Fix init_fpstate.header.xcomp_bv

On 02/22/2016 10:59 AM, Yu-cheng Yu wrote:
> This actually does not apply to XSAVES as XSAVES uses optimization. After
> init, all components are in INIT and not MODIFIED state and will not be
> saved. There is no need of setting xcomp_bv except for bit 63 to indicate
> a compacted format.

I look at it this way: xcomp_bv tells you the format of the buffer
xstate_bv tells you which components of the buffer are valid (not in
init state).

As it stands, every kernel xcomp_bv value should be identical, since the
kernel buffer is always in the same state and XSAVES is always called
with the same Requested Feature BitMap (RFBM).

While I don't think this patch _hurts_ anything, I think it might be a
bit confusing.  Wouldn't it just be easier if *EVERY* xcomp_bv value was
the same?