iommu/arm-smmu-v3: Rework enabling/disabling of ATS for PCI masters

To prevent any potential issues arising from speculative Address
Translation Requests from an ATS-enabled PCIe endpoint, rework our ATS
enabling/disabling logic so that we enable ATS at the SMMU before we
enable it at the endpoint, and disable things in the opposite order.
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

drivers/iommu/arm-smmu-v3.c

@@ -2286,44 +2286,52 @@ static void arm_smmu_install_ste_for_dev(struct arm_smmu_master *master)
 	}
 }
 
-static int arm_smmu_enable_ats(struct arm_smmu_master *master)
+static bool arm_smmu_ats_supported(struct arm_smmu_master *master)
 {
-	int ret;
-	size_t stu;
 	struct pci_dev *pdev;
 	struct arm_smmu_device *smmu = master->smmu;
 	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(master->dev);
 
 	if (!(smmu->features & ARM_SMMU_FEAT_ATS) || !dev_is_pci(master->dev) ||
 	    !(fwspec->flags & IOMMU_FWSPEC_PCI_RC_ATS) || pci_ats_disabled())
-		return -ENXIO;
+		return false;
 
 	pdev = to_pci_dev(master->dev);
-	if (pdev->untrusted)
-		return -EPERM;
+	return !pdev->untrusted && pdev->ats_cap;
+}
 
-	/* Smallest Translation Unit: log2 of the smallest supported granule */
-	stu = __ffs(smmu->pgsize_bitmap);
+static void arm_smmu_enable_ats(struct arm_smmu_master *master)
+{
+	size_t stu;
+	struct pci_dev *pdev;
+	struct arm_smmu_device *smmu = master->smmu;
 
-	ret = pci_enable_ats(pdev, stu);
-	if (ret)
-		return ret;
+	/* Don't enable ATS at the endpoint if it's not enabled in the STE */
+	if (!master->ats_enabled)
+		return;
 
-	master->ats_enabled = true;
-	return 0;
+	/* Smallest Translation Unit: log2 of the smallest supported granule */
+	stu = __ffs(smmu->pgsize_bitmap);
+	pdev = to_pci_dev(master->dev);
+	if (pci_enable_ats(pdev, stu))
+		dev_err(master->dev, "Failed to enable ATS (STU %zu)\n", stu);
 }
 
 static void arm_smmu_disable_ats(struct arm_smmu_master *master)
 {
 	struct arm_smmu_cmdq_ent cmd;
 
-	if (!master->ats_enabled || !dev_is_pci(master->dev))
+	if (!master->ats_enabled)
 		return;
 
+	pci_disable_ats(to_pci_dev(master->dev));
+	/*
+	 * Ensure ATS is disabled at the endpoint before we issue the
+	 * ATC invalidation via the SMMU.
+	 */
+	wmb();
 	arm_smmu_atc_inv_to_cmd(0, 0, 0, &cmd);
 	arm_smmu_atc_inv_master(master, &cmd);
-	pci_disable_ats(to_pci_dev(master->dev));
-	master->ats_enabled = false;
 }
 
 static void arm_smmu_detach_dev(struct arm_smmu_master *master)
@@ -2338,10 +2346,10 @@ static void arm_smmu_detach_dev(struct arm_smmu_master *master)
 	list_del(&master->domain_head);
 	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
 
+	arm_smmu_disable_ats(master);
 	master->domain = NULL;
+	master->ats_enabled = false;
 	arm_smmu_install_ste_for_dev(master);
-
-	arm_smmu_disable_ats(master);
 }
 
 static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
@@ -2386,12 +2394,13 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
 	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
 
 	if (smmu_domain->stage != ARM_SMMU_DOMAIN_BYPASS)
-		arm_smmu_enable_ats(master);
+		master->ats_enabled = arm_smmu_ats_supported(master);
 
 	if (smmu_domain->stage == ARM_SMMU_DOMAIN_S1)
 		arm_smmu_write_ctx_desc(smmu, &smmu_domain->s1_cfg);
 
 	arm_smmu_install_ste_for_dev(master);
+	arm_smmu_enable_ats(master);
 out_unlock:
 	mutex_unlock(&smmu_domain->init_mutex);
 	return ret;