From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in
 copy_fpstate_to_sigframe()
Date: Fri, 18 Jan 2019 13:17:28 -0800
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Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
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        Dave Hansen <dave.hansen@linux.intel.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>
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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*.  Inspecting that state is how
userspace makes sense of MPX or protection keys faults.  We even use
this in selftests/.