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
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@ -21,7 +21,7 @@ static inline bool is_shared_processor(void)
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return static_branch_unlikely(&shared_processor);
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}
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/* If bit 0 is set, the cpu has been preempted */
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/* If bit 0 is set, the cpu has been ceded, conferred, or preempted */
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static inline u32 yield_count_of(int cpu)
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{
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__be32 yield_count = READ_ONCE(lppaca_of(cpu).yield_count);
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@ -92,6 +92,19 @@ static inline void prod_cpu(int cpu)
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#define vcpu_is_preempted vcpu_is_preempted
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static inline bool vcpu_is_preempted(int cpu)
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{
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/*
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* The dispatch/yield bit alone is an imperfect indicator of
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* whether the hypervisor has dispatched @cpu to run on a physical
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* processor. When it is clear, @cpu is definitely not preempted.
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* But when it is set, it means only that it *might* be, subject to
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* other conditions. So we check other properties of the VM and
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* @cpu first, resorting to the yield count last.
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*/
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/*
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* Hypervisor preemption isn't possible in dedicated processor
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* mode by definition.
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*/
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if (!is_shared_processor())
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return false;
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@ -100,9 +113,10 @@ static inline bool vcpu_is_preempted(int cpu)
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int first_cpu = cpu_first_thread_sibling(smp_processor_id());
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/*
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* Preemption can only happen at core granularity. This CPU
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* is not preempted if one of the CPU of this core is not
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* preempted.
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* The PowerVM hypervisor dispatches VMs on a whole core
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* basis. So we know that a thread sibling of the local CPU
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* cannot have been preempted by the hypervisor, even if it
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* has called H_CONFER, which will set the yield bit.
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*/
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if (cpu_first_thread_sibling(cpu) == first_cpu)
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return false;
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