From mboxrd@z Thu Jan  1 00:00:00 1970
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in
 copy_fpstate_to_sigframe()
Date: Mon, 21 Jan 2019 12:21:17 +0100
Message-ID: <20190121112117.GA32538@redhat.com>
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Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Rik van Riel <riel@surriel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
To: Dave Hansen <dave.hansen@intel.com>
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On 01/18, 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*.  Inspecting that state is how
> userspace makes sense of MPX or protection keys faults.  We even use
> this in selftests/.

Yes.

And in any case I do not understand the idea to use the second in-kernel struct fpu.
A signal handler can be interrupted by another signal, this will need to save/restore
the FPU state again.

Oleg.