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

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Date:   Tue, 22 Jan 2019 18:00:23 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 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...

Hohumm. Makes sense.

-- 
Regards/Gruss,
    Boris.

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