1 files changed, 35 insertions, 5 deletions

diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
index bcb7b5f917be..eb7df559ae74 100644
--- a/arch/powerpc/include/asm/paravirt.h
+++ b/arch/powerpc/include/asm/paravirt.h
@@ -21,7 +21,7 @@ static inline bool is_shared_processor(void)
 	return static_branch_unlikely(&shared_processor);
 }
 
-/* If bit 0 is set, the cpu has been preempted */
+/* If bit 0 is set, the cpu has been ceded, conferred, or preempted */
 static inline u32 yield_count_of(int cpu)
 {
 	__be32 yield_count = READ_ONCE(lppaca_of(cpu).yield_count);
@@ -92,17 +92,47 @@ static inline void prod_cpu(int cpu)
 #define vcpu_is_preempted vcpu_is_preempted
 static inline bool vcpu_is_preempted(int cpu)
 {
+	/*
+	 * The dispatch/yield bit alone is an imperfect indicator of
+	 * whether the hypervisor has dispatched @cpu to run on a physical
+	 * processor. When it is clear, @cpu is definitely not preempted.
+	 * But when it is set, it means only that it *might* be, subject to
+	 * other conditions. So we check other properties of the VM and
+	 * @cpu first, resorting to the yield count last.
+	 */
+
+	/*
+	 * Hypervisor preemption isn't possible in dedicated processor
+	 * mode by definition.
+	 */
 	if (!is_shared_processor())
 		return false;
 
 #ifdef CONFIG_PPC_SPLPAR
 	if (!is_kvm_guest()) {
-		int first_cpu = cpu_first_thread_sibling(smp_processor_id());
+		int first_cpu;
 
 		/*
-		 * Preemption can only happen at core granularity. This CPU
-		 * is not preempted if one of the CPU of this core is not
-		 * preempted.
+		 * The result of vcpu_is_preempted() is used in a
+		 * speculative way, and is always subject to invalidation
+		 * by events internal and external to Linux. While we can
+		 * be called in preemptable context (in the Linux sense),
+		 * we're not accessing per-cpu resources in a way that can
+		 * race destructively with Linux scheduler preemption and
+		 * migration, and callers can tolerate the potential for
+		 * error introduced by sampling the CPU index without
+		 * pinning the task to it. So it is permissible to use
+		 * raw_smp_processor_id() here to defeat the preempt debug
+		 * warnings that can arise from using smp_processor_id()
+		 * in arbitrary contexts.
+		 */
+		first_cpu = cpu_first_thread_sibling(raw_smp_processor_id());
+
+		/*
+		 * The PowerVM hypervisor dispatches VMs on a whole core
+		 * basis. So we know that a thread sibling of the local CPU
+		 * cannot have been preempted by the hypervisor, even if it
+		 * has called H_CONFER, which will set the yield bit.
 		 */
 		if (cpu_first_thread_sibling(cpu) == first_cpu)
 			return false;