Commit 799f9b51 authored by Nathan Lynch's avatar Nathan Lynch Committed by Michael Ellerman
Browse files

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: default avatarNathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: default avatarSrikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210928214147.312412-2-nathanl@linux.ibm.com
parent 56537faf

arch/powerpc/include/asm/paravirt.h

+18 −4
Original line number Diff line number Diff line
@@ -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;