On 2021-11-16 09:06, Yicong Yang via iommu wrote:
[...]
+/*
+ * Get RMR address if provided by the firmware.
+ * Return 0 if the IOMMU doesn't present or the policy of the
+ * IOMMU domain is passthrough or we get a usable RMR region.
+ * Otherwise a negative value is returned.
+ */
+static int hisi_ptt_get_rmr(struct hisi_ptt *hisi_ptt)
+{
+ struct pci_dev *pdev = hisi_ptt->pdev;
+ struct iommu_domain *iommu_domain;
+ struct iommu_resv_region *region;
+ LIST_HEAD(list);
+
+ /*
+ * Use direct DMA if IOMMU does not present or the policy of the
+ * IOMMU domain is passthrough.
+ */
+ iommu_domain = iommu_get_domain_for_dev(&pdev->dev);
+ if (!iommu_domain || iommu_domain->type == IOMMU_DOMAIN_IDENTITY)
+ return 0;
+
+ iommu_get_resv_regions(&pdev->dev, &list);
+ list_for_each_entry(region, &list, list)
+ if (region->type == IOMMU_RESV_DIRECT &&
+ region->length >= HISI_PTT_TRACE_BUFFER_SIZE) {
+ hisi_ptt->trace_ctrl.has_rmr = true;
+ hisi_ptt->trace_ctrl.rmr_addr = region->start;
+ hisi_ptt->trace_ctrl.rmr_length = region->length;
+ break;
+ }
+
+ iommu_put_resv_regions(&pdev->dev, &list);
+ return hisi_ptt->trace_ctrl.has_rmr ? 0 : -ENOMEM;
+}
No.
The whole point of RMRs is for devices that are already configured to
access the given address range in a manner beyond the kernel's control.
If you can do this, it proves that you should not have an RMR in the
first place.
The notion of a kernel driver explicitly configuring its device to DMA
into any random RMR that looks big enough is so egregiously wrong that
I'm almost lost for words...
Robin.
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