[PATCH] iommu/arm-smmu-v3: add nr_ats_masters to avoid unnecessary operations

[Zhen Lei]

When (smmu_domain->smmu->features & ARM_SMMU_FEAT_ATS) is true, even if a
smmu domain does not contain any ats master, the operations of
arm_smmu_atc_inv_to_cmd() and lock protection in arm_smmu_atc_inv_domain()
are always executed. This will impact performance, especially in
multi-core and stress scenarios. For my FIO test scenario, about 8%
performance reduced.

In fact, we can use a atomic member to record how many ats masters the
smmu contains. And check that without traverse the list and check all
masters one by one in the lock protection.

Fixes: 9ce27afc0830 ("iommu/arm-smmu-v3: Add support for PCI ATS")
Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
---
 drivers/iommu/arm-smmu-v3.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index a9a9fabd396804a..1b370d9aca95f94 100644
--- a/drivers/iommu/arm-smmu-v3.c
+++ b/drivers/iommu/arm-smmu-v3.c
@@ -631,6 +631,7 @@ struct arm_smmu_domain {
 
 	struct io_pgtable_ops		*pgtbl_ops;
 	bool				non_strict;
+	atomic_t			nr_ats_masters;
 
 	enum arm_smmu_domain_stage	stage;
 	union {
@@ -1531,7 +1532,7 @@ static int arm_smmu_atc_inv_domain(struct arm_smmu_domain *smmu_domain,
 	struct arm_smmu_cmdq_ent cmd;
 	struct arm_smmu_master *master;
 
-	if (!(smmu_domain->smmu->features & ARM_SMMU_FEAT_ATS))
+	if (!atomic_read(&smmu_domain->nr_ats_masters))
 		return 0;
 
 	arm_smmu_atc_inv_to_cmd(ssid, iova, size, &cmd);
@@ -1869,6 +1870,7 @@ static int arm_smmu_enable_ats(struct arm_smmu_master *master)
 	size_t stu;
 	struct pci_dev *pdev;
 	struct arm_smmu_device *smmu = master->smmu;
+	struct arm_smmu_domain *smmu_domain = master->domain;
 	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(master->dev);
 
 	if (!(smmu->features & ARM_SMMU_FEAT_ATS) || !dev_is_pci(master->dev) ||
@@ -1887,12 +1889,15 @@ static int arm_smmu_enable_ats(struct arm_smmu_master *master)
 		return ret;
 
 	master->ats_enabled = true;
+	atomic_inc(&smmu_domain->nr_ats_masters);
+
 	return 0;
 }
 
 static void arm_smmu_disable_ats(struct arm_smmu_master *master)
 {
 	struct arm_smmu_cmdq_ent cmd;
+	struct arm_smmu_domain *smmu_domain = master->domain;
 
 	if (!master->ats_enabled || !dev_is_pci(master->dev))
 		return;
@@ -1901,6 +1906,7 @@ static void arm_smmu_disable_ats(struct arm_smmu_master *master)
 	arm_smmu_atc_inv_master(master, &cmd);
 	pci_disable_ats(to_pci_dev(master->dev));
 	master->ats_enabled = false;
+	atomic_dec(&smmu_domain->nr_ats_masters);
 }
 
 static void arm_smmu_detach_dev(struct arm_smmu_master *master)
@@ -1915,10 +1921,10 @@ static void arm_smmu_detach_dev(struct arm_smmu_master *master)
 	list_del(&master->domain_head);
 	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
 
-	master->domain = NULL;
 	arm_smmu_install_ste_for_dev(master);
 
 	arm_smmu_disable_ats(master);
+	master->domain = NULL;
 }
 
 static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
-- 
1.8.3