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Date: Thu, 6 Aug 2015 10:29:09 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Anvin <hpa@zytor.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>, bp@alien8.de,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, fenghua.yu@intel.com,
        x86@kernel.org, dave.hansen@linux.intel.com,
        Dave Hansen <dave@sr71.net>
Subject: Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation
Message-ID: <20150806082909.GA13169@gmail.com>
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* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > On Aug 6, 2015 10:15 AM, "Ingo Molnar" <mingo@kernel.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