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Date:   Fri, 18 Jan 2019 22:37:17 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>, linux-kernel@vger.kernel.org,
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        Rik van Riel <riel@surriel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in
 copy_fpstate_to_sigframe()
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On 2019-01-18 13:17:28 [-0800], Dave Hansen wrote:
> On 1/18/19 1:14 PM, Sebastian Andrzej Siewior wrote:
> > The kernel saves task's FPU registers on user's signal stack before
> > entering the signal handler. Can we avoid that and have in-kernel memory
> > for that? Does someone rely on the FPU registers from the task in the
> > signal handler?
> 
> This is part of our ABI for *sure*.  

I missed that part. I will try to look it up and look see if says
something about optional part.
But ABI means we must keep doing it even if there are no users?

> Inspecting that state is how
> userspace makes sense of MPX or protection keys faults.  We even use
> this in selftests/.

Okay. MPX does not check for FP_XSTATE_MAGIC[12] and simply assumes it
is there. That is why I didn't find it.
So we would break MPX. But then MPX is on its way out, so…

Sebastian