[PATCH v2 1/2] powerpc/paravirt: vcpu_is_preempted() commentary

[Srikar Dronamraju]

* Nathan Lynch <nathanl at linux.ibm.com> [2021-09-28 16:41:46]:

> Add comments more clearly documenting that this function determines whether
> hypervisor-level preemption of the VM has occurred.
> 
> Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>

Looks good to me.

Reviewed-by: Srikar Dronamraju <srikar at linux.vnet.ibm.com>

> ---
>  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
> 

-- 
Thanks and Regards
Srikar Dronamraju