Messages in this thread | | | Date | Tue, 17 Nov 2015 01:04:10 +0100 | Subject | Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets | From | "Jason A. Donenfeld" <> |
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On Tue, Nov 17, 2015 at 12:57 AM, Jason A. Donenfeld <Jason@zx2c4.com> 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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