linux-kernel - Re: [PATCH 084/208] x86/fpu: Rename xsave.header::xstate

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Date:	Tue, 05 May 2015 11:25:35 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 084/208] x86/fpu: Rename xsave.header::xstate_bv to 'xfeatures'

On 05/05/2015 11:16 AM, Ingo Molnar wrote:
> We could put the SDM name into a comment, next to the field 
> definition? Something like, if 'xfeatures' is too long:
> 
> struct xstate_header {
>         u64     xfeat;        /* xstate components,           SDM: XSTATE_BV */
>         u64     xfeat_comp;   /* compacted xstate components, SDM: XCOMP_BV */
>         u64     reserved[6];
> } __attribute__((packed));

When you're in the depths of the SDM and the kernel code, the fewer
context switches you have to make, the better.  I say this from the
perspective of someone who's had a copy of the SDM open to xsave* for
about a month straight.

In any case, having "xfeat" and "xfeat_comp" is a bad idea.  They're not
really related concepts other than their bits refer to the same states.
 They should not have such similar names.

XSTATE_BV is the set of states written to the xsave area.

XCOMP_BV is essentially always XCR0 (aka pcntxt_mask, aka
xfeatures_mask) or'd with bit 63.
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