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

[John Garry <john.garry@huawei.com>]

On 05/08/2021 12:24, Robin Murphy wrote:
> 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.

I think that the problem is if the prod+cons 64b value and the shift are 
on the same cacheline, then we have a chance of accessing a stale 
cacheline twice:

static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
				       u64 *cmds, int n, bool sync)
{
	u64 cmd_sync[CMDQ_ENT_DWORDS];
	u32 prod;
	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,	// here
	}, head = llq;
	int ret = 0;

	/* 1. Allocate some space in the queue */
	local_irq_save(flags);
	llq.val = READ_ONCE(cmdq->q.llq.val);	// and again here


since cmdq->q.llq is per-SMMU. If max_n_shift is on a separate 
cacheline, then it should never be stale.

I suppose they could be combined into a smaller sub-struct and loaded in 
a single operation, but it looks messy, and prob without much gain.

Thanks,
John

> 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);
>>
> .


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