linux-kernel - Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in copy_fpstate_to

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Date:   Fri, 18 Jan 2019 13:43:17 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in
 copy_fpstate_to_sigframe()

On 1/18/19 1:37 PM, Sebastian Andrzej Siewior wrote:
> 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?

I'd bet a large sum of money there are users.

Google for 'uc_mcontext fpregs'.