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.mur...@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.ga...@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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