Re: [v1][PATCH 1/1] KVM: PPC: disable preemption when using hard_irq_disable()

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2013-07-11 at 14:47 +0200, Alexander Graf wrote:
> > Yes of course, that's what we call "soft disabled" :-) It's even the
> > whole point of doing lazy disable...
> 
> Meh. Of course it's soft_enabled = 1; hard_enabled = 0.

That doesn't happen in "normal" C code. It happens under very specific
circumstances, such as in the guts of entry_64.S, in areas where we just
want to temporarily mask HW interrupts without changing the SW state
(and thus without having to deal with replays etc...).

We typically also do that right before going to idle on some processors
where we come back from idle with interrupts hard enabled, possibly
right in an interrupt vector.

Typically that's a state that makes some sense on KVM entry. From a
Linux perspective, you enter KVM with interrupts enabled. But you
temporarily hard disable on the way down while doing the low level
context switch.

This works well as long as you know you have no pending replay event.
That should be fine if you do a direct transition from soft+hard enabled
to hard disabled (without soft disabling). In that case there should be
nothing in irq_happened.

It's equivalent to returning to userspace from the kernel. We are
soft-enabled, but the code in ret_from_except hard disables while
mucking around with TIF_FLAGS etc... until the final rfid

Cheers,
Ben.