[PATCH] powerpc/pseries: unregister VPA when hot unplugging a CPU

[Laurent Dufour <ldufour@linux.ibm.com>]

The VPA should unregister when offlining a CPU. Otherwise there could be a
short window where 2 CPUs could share the same VPA.

This happens because the hypervisor is still keeping the VPA attached to
the vCPU even if it became offline.

Here is a potential situation:
 1. remove proc A,
 2. add proc B. If proc B gets proc A's place in cpu_present_map, then it
    registers proc A's VPAs.
 3. If proc B is then re-added to the LP, its threads are sharing VPAs with
    proc A briefly as they come online.

As the hypervisor may check for the VPA's yield_count field oddity, it may
detects an unexpected value and kill the LPAR.

Suggested-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
---
 arch/powerpc/platforms/pseries/hotplug-cpu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index e0a7ac5db15d..090ae5a1e0f5 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -70,6 +70,7 @@ static void pseries_cpu_offline_self(void)
 		xics_teardown_cpu();
 
 	unregister_slb_shadow(hwcpu);
+	unregister_vpa(hwcpu);
 	rtas_stop_self();
 
 	/* Should never get here... */
-- 
2.38.1