netdev - Re: irq_fpu_usable() is false in ndo_start

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Message-Id: <1447712830.2096939.441563921.3680DBCF@webmail.messagingengine.com>
Date:	Mon, 16 Nov 2015 23:27:10 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	"Jason A. Donenfeld" <Jason@...c4.com>,
	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 Jason,

On Mon, Nov 16, 2015, at 21:58, Jason A. Donenfeld wrote:
> 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();
> }

Use the irqsoff tracer to find the problematic functions which disable
interrupts and try to work around it in case of UDP. This could benefit
the whole stack.

Bye,
Hannes
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