Re: irq_fpu_usable() is irreliable

[Jason A. Donenfeld]

Hi Thomas,

On Tue, Nov 17, 2015 at 3:06 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> The real question in your case is WHY interrupted_kernel_fpu_idle()
> returns false. We know for sure that in a syscall with BH disabled the
> first two checks are false.

Blurg, indeed. I've been trying to track that down in a different
thread with the netdev folks. After not getting anywhere, I sort of
felt, "dammit! can't this not be the issue, and can't I just get rid
of that in_interrupt() condition?" But, as you've explained above, no,
we can't get rid of that. So yes: the question is why
interrupted_kernel_fpu_idle() is false. Mysteriously it happens to be
the case in UDP mode but not TCP mode (the topic of the other thread),
and so I should resume trying to determine why this is so. I don't
entirely understand the function though:

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);
}

>From my tests, when irq_fpu_usable() is false, the expression
`!current->thread.fpu.fpregs_active && (read_cr0() & X86_CR0_TS);` is
false, for both of it. What, then, is leading to the call of
fpregs_activate()? I can't find anything along the syscall path that
would result in this. I admit I do not have a deep understanding of
how the FPU is implemented in Linux. Is it possible that this means
that userspace is using the FPU? Is this what user_fpu_begin() is all
about?

(If so, why is that state not stored on syscall entry? If the reason
is "because it would be expensive to do it everytime", then is there a
way to selectively do that only when it's necessary?)

Or, must this imply that the kernel is actually using it elsewhere,
and I need to just keep digging diligently?

Thanks,
Jason
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