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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
This commit is contained in:
Nathan Lynch 2021-09-28 16:41:46 -05:00 committed by Michael Ellerman
parent 56537faf88
commit 799f9b51db
1 changed files with 18 additions and 4 deletions
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22
arch/powerpc/include/asm/paravirt.h
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@ -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;