[pve-devel] [PATCH docs 1/2] pcie-passthrough: note that iommu activation is not always necessary

[pve-devel] [PATCH docs 1/2] pcie-passthrough: note that iommu activation is not always necessary

[Dominik Csapak]

especially on newer kernels

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
---
 qm-pci-passthrough.adoc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/qm-pci-passthrough.adoc b/qm-pci-passthrough.adoc
index fa6ba35..5ef325d 100644
--- a/qm-pci-passthrough.adoc
+++ b/qm-pci-passthrough.adoc
@@ -49,8 +49,9 @@ First, the IOMMU support has to be enabled in your BIOS/UEFI. Most often, that
 options is named `IOMMU` or `VT-d`, but check the manual for your motherboard
 for the exact option you need to enable.
 
-Then, the IOMMU has to be activated on the
+Then, the IOMMU might need to be activated on the
 xref:sysboot_edit_kernel_cmdline[kernel commandline].
+(On newer kernels, this should not be necessary.)
 
 The command line parameters are:
 
-- 
2.30.2

[pve-devel] [PATCH docs 2/2] pcie-passthrough: add short note about iommu passthrough mode

[Dominik Csapak]

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>
---
 qm-pci-passthrough.adoc | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/qm-pci-passthrough.adoc b/qm-pci-passthrough.adoc
index 5ef325d..28bb4fc 100644
--- 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
-- 
2.30.2

[pve-devel] applied: [PATCH docs 1/2] pcie-passthrough: note that iommu activation is not always necessary

[Thomas Lamprecht]

Am 20/05/2022 um 10:43 schrieb Dominik Csapak:
> especially on newer kernels
> 
> Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
> ---
>  qm-pci-passthrough.adoc | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
>

applied both patches with dropping the `iommu.passthrough=1` part in a follow up as
its only relevant for ARM64 as talked off list, thanks!