pci passthrough: rework IOMMU sections a bit

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

qm-pci-passthrough.adoc

@@ -44,34 +44,33 @@ some configuration to enable PCI(e) passthrough.
 
 .IOMMU
 
-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.
+First, you have to enable IOMMU support in your BIOS/UEFI. Usually the
+corresponding setting is called `IOMMU` or `VT-d`,but you should find the exact
+option name in the manual of your motherboard.
 
-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.)
+For Intel CPUs, you may also need to enable the IOMMU on the
+xref:sysboot_edit_kernel_cmdline[kernel command line] for older (pre-5.15)
+kernels by adding:
 
-The command line parameters are:
-
-* for Intel CPUs:
-+
 ----
  intel_iommu=on
 ----
-* for AMD CPUs it should be enabled automatically.
 
+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 Mode
+
+If your hardware supports IOMMU passthrough mode, enabling this mode might
+increase performance.
+This is because VMs then bypass the (default) DMA translation normally
+performed by the hyper-visor and instead pass DMA requests directly to the
+hardware IOMMU. To enable these options, add:
 
 ----
  iommu=pt
 ----
 
-to the kernel commandline.
+to the xref:sysboot_edit_kernel_cmdline[kernel commandline].
 
 .Kernel Modules