linux-kernel - Re: [PATCH v5 12/13] x86/xsaves: Fix fpstate

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Date:	Mon, 9 May 2016 16:41:31 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
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 v5 12/13] x86/xsaves: Fix fpstate_init() for XSAVES

On 05/09/2016 01:46 PM, Yu-cheng Yu wrote:
> When XSAVES is used, xsave.header.xcomp_bv[63] must be set.

So, that's not strictly correct.  XSAVES can write to a completely empty
(0'd) memory buffer.  It's only XRSTORS that _needs_ bit 63 set.  The
instruction reference is pretty clear on this point.

Oh, and if you decided to do this for some reason, please have mercy and
go make a macro for 1<<63.

Also, I don't think the kernel ever checks for this bit.  So are we
really calling XRSTORS on otherwise uninitialized xsave buffers?