From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: Re: [PATCH net-next v5 02/20] zinc: introduce minimal cryptography library
Date: Fri, 21 Sep 2018 06:15:46 +0200
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Cc: Andrew Lunn <andrew@lunn.ch>, Arnd Bergmann <arnd@arndb.de>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Eric Biggers <ebiggers@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        David Miller <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Samuel Neves <sneves@dei.uc.pt>,
        Andrew Lutomirski <luto@kernel.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <8FA361E3-FBCB-469E-88DC-F4085BD91175@amacapital.net>
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Hi Andy,

On Fri, Sep 21, 2018 at 5:23 AM Andy Lutomirski <luto@amacapital.net> wrote=
:
> At the risk on suggesting something awful: on x86_64, since we turn preem=
ption off for simd, it wouldn=E2=80=99t be *completely* insane to do the cr=
ypto on the irq stack. It would look like:
>
> kernel_fpu_call(func, arg);
>
> And this helper would disable preemption, enable FPU, switch to the irq s=
tack, call func(arg), disable FPU, enable preemption, and return. And we ca=
n have large IRQ stacks.
>
> I refuse to touch this with a ten-foot pole until the lazy FPU restore pa=
tches land.

Haha. That's fun, and maybe we'll do that at some point, but I have
some other reasons too for being on a workqueue now.

>
> All that being said, why are these frames so large?  It sounds like somet=
hing may be spilling that ought not to.

They're not. Well, they're not anymore. I had a silly thing before
like "u8 buffer[1 << 12]" in some debugging code, which is what
prompted the ccflag-y addition. I cleaned up the mistakes like that
and frames are now reasonable everywhere. Non-issue.

Jason