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

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Message-ID: <20160509235042.GB28442@test-lenovo>
Date:	Mon, 9 May 2016 16:50:42 -0700
From:	Yu-cheng Yu <yu-cheng.yu@...el.com>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	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 Mon, May 09, 2016 at 04:41:31PM -0700, Dave Hansen wrote:
> 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.

You mean the comments?  I will change it to XRSTORS.

> 
> 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?

Right now if we do fpstate_init(), without this patch, it will 
trigger a warning from copy_kernel_to_fxregs() when the task is
scheduled.