From 799f9b51db688608b50e630a57bee5f699b268ca Mon Sep 17 00:00:00 2001
From: Nathan Lynch <nathanl@linux.ibm.com>
Date: Tue, 28 Sep 2021 16:41:46 -0500
Subject: [PATCH] powerpc/paravirt: vcpu_is_preempted() commentary
References: bsc#1181148 ltc#190702 git-fixes
Patch-mainline: v5.16-rc1
Git-commit: 799f9b51db688608b50e630a57bee5f699b268ca
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
Acked-by: Michal Suchanek <msuchanek@suse.de>
---
arch/powerpc/include/asm/paravirt.h | 22 ++++++++++++++++++----
1 file changed, 18 insertions(+), 4 deletions(-)
diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
index bcb7b5f917be..39f173961f6a 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);
@@ -92,6 +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;
--
2.31.1