linux-kernel - Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation

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Message-ID: <20150806082909.GA13169@gmail.com>
Date:	Thu, 6 Aug 2015 10:29:09 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Anvin <hpa@...or.com>, Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
	bp@...en8.de, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	fenghua.yu@...el.com, x86@...nel.org, dave.hansen@...ux.intel.com,
	Dave Hansen <dave@...1.net>
Subject: Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Aug 6, 2015 10:15 AM, "Ingo Molnar" <mingo@...nel.org> wrote:
> > >
> > > What sense does it make to have a blob we don't know the exact layout of? How 
> > > will debuggers or user-space in general be able to print (and change) the 
> > > register values if they don't know the layout?
> > 
> > The usage model is that you only use this for saving and restoring state.
> > 
> > If you look at the state, you restore the state and then you look at the 
> > registers. You never look at the blob itself.
> 
> So we are relying on the saved structure already in a couple of cases, such as MPX 
> exception handling:
> 
>         /*
>          * We need to look at BNDSTATUS to resolve this exception.
>          * A NULL here might mean that it is in its 'init state',
>          * which is all zeros which indicates MPX was not
>          * responsible for the exception.
>          */
>         bndcsr = get_xsave_field_ptr(XSTATE_BNDCSR);
>         if (!bndcsr)
>                 goto exit_trap;
> 
>         trace_bounds_exception_mpx(bndcsr);
> 
> get_xsave_field_ptr() very much knows about the structure.

Correction:

  get_xsave_field_ptr() users very much know about the structure.

Thanks,

	Ingo
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