Re: [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist()

[Robin Murphy <robin.murphy@arm.com>]

On 2021-06-21 17:36, John Garry wrote:
> Members of struct "llq" will be zero-inited, apart from member max_n_shift.
> But we write llq.val straight after the init, so it was pointless to zero
> init those other members. As such, separately init member max_n_shift
> only.
> 
> In addition, struct "head" is initialised to "llq" only so that member
> max_n_shift is set. But that member is never referenced for "head", so
> remove any init there.
> 
> Removing these initializations is seen as a small performance optimisation,
> as this code is (very) hot path.

I looked at this and immediately thought "surely the compiler can see 
that all the prod/cons/val fields are written anyway and elide the 
initialisation?", so I dumped the before and after disassembly, and... oh.

You should probably clarify that it's zero-initialising all the 
cacheline padding which is both pointless and painful. With that,

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

However, having looked this closely I'm now tangentially wondering why 
max_n_shift isn't inside the padded union? It's read at the same time as 
both prod and cons by queue_has_space(), and never updated, so there 
doesn't appear to be any benefit to it being in a separate cacheline all 
by itself, and llq is already twice as big as it needs to be. Sorting 
that would also be a good opportunity to store the value of interest in 
its appropriate form so we're not needlessly recalculating 1 << shift 
every flippin' time...

Robin.

> Signed-off-by: John Garry <john.garry@huawei.com>
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 54b2f27b81d4..8a8ad49bb7fd 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -727,11 +727,11 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
>   	unsigned long flags;
>   	bool owner;
>   	struct arm_smmu_cmdq *cmdq = &smmu->cmdq;
> -	struct arm_smmu_ll_queue llq = {
> -		.max_n_shift = cmdq->q.llq.max_n_shift,
> -	}, head = llq;
> +	struct arm_smmu_ll_queue llq, head;
>   	int ret = 0;
>   
> +	llq.max_n_shift = cmdq->q.llq.max_n_shift;
> +
>   	/* 1. Allocate some space in the queue */
>   	local_irq_save(flags);
>   	llq.val = READ_ONCE(cmdq->q.llq.val);
>