pcie-passthrough: add short note about iommu passthrough mode

that explains in one sentence what it does, that it can increase
performance and how to activate it.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

diff --git a/qm-pci-passthrough.adoc b/qm-pci-passthrough.adoc
index 5ef325d38140d1f1863aa6a9a22bbccff291fae8..28bb4fc39dff30b3b8b2ff4e6b1846b5a4e512cb 100644 (file)
--- a/qm-pci-passthrough.adoc
+++ b/qm-pci-passthrough.adoc
@@ -62,6 +62,24 @@ The command line parameters are:
 ----
 * for AMD CPUs it should be enabled automatically.
 
+
+If your hardware supports it, enabling IOMMU passthrough mode might increase
+performance, because then the VMs bypass the (default) DMA translation
+which is normally done by the hypervisor, before handing DMA requests off to
+the hardware IOMMU. You can enable it with adding
+
+----
+ iommu.passthrough=1
+----
+
+or
+
+----
+ iommu=pt
+----
+
+to the kernel commandline.
+
 .Kernel Modules
 
 You have to make sure the following modules are loaded. This can be achieved by