Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets

From: Jason A. Donenfeld
Date: Mon Nov 16 2015 - 19:04:20 EST

On Tue, Nov 17, 2015 at 12:57 AM, Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> 2. irq_fpu_usable() is FALSE for UDP! Since in_interrupt() is always
> true in ndo_start_xmit, this means that in this case, both
> interrupted_user_mode() and interrupted_kernel_fpu_idle() are false!
> Investigating further, will report back.

GASP, the plot thickens.

It turns out that when it is working, for TCP, interrupted_user_mode()
is false. This means that the only reason it's succeeding for TCP is
because interrupted_kernel_fpu_idle() is true. Therefore, for some
reason for UDP, interrupted_kernel_fpu_idle() is false!

That function is defined as:

static bool interrupted_kernel_fpu_idle(void)
if (kernel_fpu_disabled())
return false;

if (use_eager_fpu())
return true;

return !current->thread.fpu.fpregs_active && (read_cr0() & X86_CR0_TS);

So now the big question is: what in the UDP pipeline is using the FPU?
And why is that usage not being released by the time it gets to
ndo_start_xmit? Or, alternatively, why would X86_CR0_TS be unset in
the UDP path? Is it possible this is related to UFO?
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