linux-kernel - Re: [PATCH v4 10/10] x86/xsaves: Re-enable XSAVES

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Date:	Wed, 4 May 2016 15:46:30 -0700
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 v4 10/10] x86/xsaves: Re-enable XSAVES

On Wed, May 04, 2016 at 03:41:49PM -0700, Dave Hansen wrote:
> It's my fault, but you also need to go update
> 	
> 	fpu__xfeature_set_state()
> and
> 	__raw_xsave_addr()
> 
> The theoretical problem is that you might ask for a __raw_xsave_addr()
> of a component which has been compacted out of an XSAVES buffer and thus
> has no address.  We could work around this by doing a memmove() and
> moving the components "up" after the one we are trying to set in order
> to make space.
> 
> But, since we *always* call XSAVES with an instruction mask of -1 and
> end up with a requested feature bitmap (RFBM) equal to XCR0, I think we
> can do a shortcut because we'll practically *always* have an
> xcomp_bv==RFBM==XCR0, which means that all (present) components will
> always have an address.
> 
> So, the alternative to doing the memmove() is to add some WARN_ON_FPU()
> checks to enforce xcomp_bv==RFBM==XCR0 in places where we call
> XSAVES/XRSTORS and __raw_xsave_addr(), maybe more.

In the coming version 5 patches, we are going to have one additional 
patch for fixing __fpu_restore_sig() for the compacted format.  I changed
my existing patch a little and run into some problems. Fixing it now.

Our ptrace tests went OK before, but are failing now. It might be relating
to what you are saying? I will check it.