Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 4 Dec 2018 13:33:55 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        the arch/x86 maintainers <x86@kernel.org>, nstange@suse.de,
        Rik van Riel <riel@surriel.com>,
        KVM devel mailing list <kvm@vger.kernel.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        linux-efi <linux-efi@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/fpu] x86/fpu: Don't export __kernel_fpu_{begin,end}()
Message-ID: <20181204123355.GD11803@zn.tnic>
References: <20181129150210.2k4mawt37ow6c2vq@linutronix.de>
 <tip-7d79adb87fa79e4a4c59424fd5b5a922861fc5f6@git.kernel.org>
 <CAKv+Gu-s59fSWrMevmbJWyBtNVYLkF7FATALGBD470CWsN9C1Q@mail.gmail.com>
 <20181203220841.GB4794@zn.tnic>
 <20181204121510.ebw4vb265lcepa2m@linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20181204121510.ebw4vb265lcepa2m@linutronix.de>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Dec 04, 2018 at 01:15:10PM +0100, Sebastian Andrzej Siewior wrote:
> Do you refer to
> | *   - by IRQ context code to potentially use the FPU
> | *     if it's unused.
>   
> ? It is possible to use the FPU in IRQ context.

I mean interrupted_user_mode() where we apparently can use the FPU when
handling an IRQ from user mode.

> The FPU could be used in user-context surrounded by kernel_fpu_begin().

Right, that.

> This only disables preemption so an IRQ could interrupt it. This IRQ
> could then use the FPU or raise a SoftIRQ which would use it.
> Therefore on x86 it is required to check with irq_fpu_usable() if the

Yes, and the check that thing does is:

        return !in_interrupt() ||
                interrupted_user_mode() || ...

so you're either *not* in interrupt, or you've gotten the IRQ while in
user mode.

> FPU can be used. If the FPU can not be used, you have to implement
> fallback code.
> 
> With the "restore FPU on return to userland" series we need to modify
> the FPU in a few places. The softirq and preemption is disabled. I
> didn't find any in-IRQ users.
> Going forward I would like to remove the in-IRQ part and
> irq_fpu_usable() and disable softirq as part of kernel_fpu_begin().

Right, and we should document all those new conditions prominently so
that people are aware.

Thx.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.