On Thu, 2013-07-11 at 15:07 +0200, Alexander Graf wrote:
> Ok, let me quickly explain the problem.
>
> We are leaving host context, switching slowly into guest context.
> During that transition we call get_paca() indirectly (apparently by
> another call to hard_disable() which sounds bogus, but that's another
> story).
>
> get_paca() warns when we're preemptible. We're only not preemptible
> when either preempt is disabled or irqs are disabled. Irqs are
> disabled, but arch_irqs_disabled() doesn't know, because it only
> checks for soft disabled IRQs.
>
> So we can fix this either by setting IRQs as soft disabled as well or
> by disabling preemption until we enter the guest for real. Any
> preferences?
Well, if you hard disable first (ie, direct transition from full enabled
to hard disabled), you know you have nothing lazy pending in
irq_pending, then it's ok to mess around with local_paca->soft_enabled
to make it "look disabled".
IE. Call hard_irq_disable(), then only turn local_paca->soft_enabled
back on late in the process, some time before the final rfi(d).
That works as long as you had a transition from full enabled to full
disabled and don't hard re-enable in the process. IE, You are certain
that there is nothing pending in irq_happened.
HOWEVER !
If you do that, you *ALSO* need to clear irq_happened. You must *NEVER*
leave PACA_IRQ_HARD_DIS set in irq_happened if you are soft-enabled, and
since the above means that you *will* be seen as soft enabled on the way
out of the guest, you can kaboom.
BTW. I'm fine with a patch that does:
#define hard_irq_disable() do { \
u8 _was_enabled = get_paca()->soft_enabled; \
__hard_irq_disable(); \
- get_paca()->soft_enabled = 0; \
+ local_paca->soft_enabled = 0; \
In fact we should probably do it anyway.
Cheers,
Ben.
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