linux-kernel - Re: irq_fpu_usable() is false in ndo_start

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Message-ID: <CAHmME9o3s5iTZ4gcFVQ1xTu9PaaLjkEGcvhJRF+S=0GugxhQ8Q@mail.gmail.com>
Date:	Mon, 16 Nov 2015 21:58:49 +0100
From:	"Jason A. Donenfeld" <Jason@...c4.com>
To:	David Miller <davem@...emloft.net>
Cc:	Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets

Hi David,

On Mon, Nov 16, 2015 at 9:32 PM, David Miller <davem@...emloft.net> wrote:
> Network device driver transmit executes with software interrupts
> disabled.
>
> Therefore on x86, you cannot use the FPU.

That is extremely problematic for me. Is there a way to make this not
so? A driver flag that would allow this?

Also - how come it irq_fpu_usable() is true when using TCP but not
when using UDP?

Further, irq_fpu_usable() doesn't only check for interrupts. There are
two other conditions that allow the FPU's usage, from
arch/x86/kernel/fpu/core.c:

bool irq_fpu_usable(void)
{
        return !in_interrupt() ||
                interrupted_user_mode() ||
                interrupted_kernel_fpu_idle();
}
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