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

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Date:   Tue, 5 Feb 2019 12:34:51 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...nel.org>, 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>,
        Michael Matz <matz@...e.de>
Subject: Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in
 copy_fpstate_to_sigframe()

On 2019-01-22 18:00:23 [+0100], Borislav Petkov wrote:
> On Tue, Jan 22, 2019 at 05:15:51PM +0100, Oleg Nesterov wrote:
> > I don't know... tried to google, found nothing.
> > 
> > the comment in /usr/include/sys/ucontext.h mentions SysV/i386 ABI + historical
> > reasons, this didn't help.
> 
> So I'm being told by one of the psABI folks that this is not really
> written down somewhere explicitly but it is the result from the POSIX
> and psABI treatise of signal handlers, what they're supposed to do,
> caller- and callee-saved registers, etc.
> 
> And FPU registers are volatile, i.e., caller-saved. Which means, the
> handler itself doesn't save them but the caller, which, doesn't really
> expect any signals - they are async. So the kernel must do that and
> slap the FPU regs onto the user stack...

My point was save them somewhere else if it is possible. So we could
save a few cycles during signal delivery and it would make the code a
little simpler.

Let me finish the series and then we can think how we could improve it.

> Hohumm. Makes sense.
> 

Sebastian