ppc/xive: Update the state of the External interrupt signal

ppc/xive: Update the state of the External interrupt signal

When pulling or pushing an OS context from/to a CPU, we should
re-evaluate the state of the External interrupt signal. Otherwise, we
can end up catching the External interrupt exception in hypervisor
mode, which is unexpected.

The problem is best illustrated with the following scenario:

1. an External interrupt is raised while the guest is on the CPU.

2. before the guest can ack the External interrupt, an hypervisor
interrupt is raised, for example the Hypervisor Decrementer or
Hypervisor Virtualization interrupt. The hypervisor interrupt forces
the guest to exit while the External interrupt is still pending.

3. the hypervisor handles the hypervisor interrupt. At this point, the
External interrupt is still pending. So it's very likely to be
delivered while the hypervisor is running. That's unexpected and can
result in an infinite loop where the hypervisor catches the External
interrupt, looks for an interrupt in its hypervisor queue, doesn't
find any, exits the interrupt handler with the External interrupt
still raised, repeat...

The fix is simply to always lower the External interrupt signal when
pulling an OS context. It means it needs to be raised again when
re-pushing the OS context. Fortunately, it's already the case, as we
now always call xive_tctx_ipb_update(), which will raise the signal if
needed.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20220429071620.177142-3-fbarrat@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>