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

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Date:	Mon, 22 Feb 2016 12:42:57 -0800
From:	Yu-cheng Yu <yu-cheng.yu@...el.com>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	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 Mon, Feb 22, 2016 at 11:55:50AM -0800, Dave Hansen wrote:
> 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?

Agree. This patch can be skipped if it turns out not needed.

--Yu-cheng