powerpc/paravirt: vcpu_is_preempted() commentary

Add comments more clearly documenting that this function determines whether
hypervisor-level preemption of the VM has occurred.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210928214147.312412-2-nathanl@linux.ibm.com

diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
index bcb7b5f..39f1739 100644 (file)
--- 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);
@@ -92,6 +92,19 @@ 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;
 
@@ -100,9 +113,10 @@ static inline bool vcpu_is_preempted(int cpu)
                int first_cpu = cpu_first_thread_sibling(smp_processor_id());
 
                /*
-                * Preemption can only happen at core granularity. This CPU
-                * is not preempted if one of the CPU of this core is not
-                * preempted.
+                * 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;