From 732de3e72d8872b34b82afb3a5b063b2e0c1c85b Mon Sep 17 00:00:00 2001 From: Xingang Wang Date: Thu, 8 Jul 2021 12:55:19 +0000 Subject: [PATCH] docs: Add documentation for iommu bypass Signed-off-by: Xingang Wang Message-Id: <1625748919-52456-10-git-send-email-wangxingang5@huawei.com> Reviewed-by: Michael S. Tsirkin Signed-off-by: Michael S. Tsirkin --- docs/bypass-iommu.txt | 89 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 89 insertions(+) create mode 100644 docs/bypass-iommu.txt diff --git a/docs/bypass-iommu.txt b/docs/bypass-iommu.txt new file mode 100644 index 0000000000..e6677bddd3 --- /dev/null +++ b/docs/bypass-iommu.txt @@ -0,0 +1,89 @@ +BYPASS IOMMU PROPERTY +===================== + +Description +=========== +Traditionally, there is a global switch to enable/disable vIOMMU. All +devices in the system can only support go through vIOMMU or not, which +is not flexible. We introduce this bypass iommu property to support +coexist of devices go through vIOMMU and devices not. This is useful to +passthrough devices with no-iommu mode and devices go through vIOMMU in +the same virtual machine. + +PCI host bridges have a bypass_iommu property. This property is used to +determine whether the devices attached on the PCI host bridge will bypass +virtual iommu. The bypass_iommu property is valid only when there is a +virtual iommu in the system, it is implemented to allow some devices to +bypass vIOMMU. When bypass_iommu property is not set for a host bridge, +the attached devices will go through vIOMMU by default. + +Usage +===== +The bypass iommu feature support PXB host bridge and default main host +bridge, we add a bypass_iommu property for PXB and default_bus_bypass_iommu +for machine. Note that default_bus_bypass_iommu is available only when +the 'q35' machine type on x86 architecture and the 'virt' machine type +on AArch64. Other machine types do not support bypass iommu for default +root bus. + +1. The following is the bypass iommu options: + (1) PCI expander bridge + qemu -device pxb-pcie,bus_nr=0x10,addr=0x1,bypass_iommu=true + (2) Arm default host bridge + qemu -machine virt,iommu=smmuv3,default_bus_bypass_iommu=true + (3) X86 default root bus bypass iommu: + qemu -machine q35,default_bus_bypass_iommu=true + +2. Here is the detailed qemu command line for 'virt' machine with PXB on +AArch64: + +qemu-system-aarch64 \ + -machine virt,kernel_irqchip=on,iommu=smmuv3,default_bus_bypass_iommu=true \ + -device pxb-pcie,bus_nr=0x10,id=pci.10,bus=pcie.0,addr=0x3.0x1 \ + -device pxb-pcie,bus_nr=0x20,id=pci.20,bus=pcie.0,addr=0x3.0x2,bypass_iommu=true \ + +And we got: + - a default host bridge which bypass SMMUv3 + - a pxb host bridge which go through SMMUv3 + - a pxb host bridge which bypass SMMUv3 + +3. Here is the detailed qemu command line for 'q35' machine with PXB on +x86 architecture: + +qemu-system-x86_64 \ + -machine q35,accel=kvm,default_bus_bypass_iommu=true \ + -device pxb-pcie,bus_nr=0x10,id=pci.10,bus=pcie.0,addr=0x3 \ + -device pxb-pcie,bus_nr=0x20,id=pci.20,bus=pcie.0,addr=0x4,bypass_iommu=true \ + -device intel-iommu \ + +And we got: + - a default host bridge which bypass iommu + - a pxb host bridge which go through iommu + - a pxb host bridge which bypass iommu + +Limitations +=========== +There might be potential security risk when devices bypass iommu, because +devices might send malicious dma request to virtual machine if there is no +iommu isolation. So it would be necessary to only bypass iommu for trusted +device. + +Implementation +============== +The bypass iommu feature includes: + - Address space + Add bypass iommu property check of PCI Host and do not get iommu address + space for devices bypass iommu. + - Arm SMMUv3 support + We traverse all PCI root bus and get bus number ranges, then build explicit + RID mapping for devices which do not bypass iommu. + - X86 IOMMU support + To support Intel iommu, we traverse all PCI host bridge and get information + of devices which do not bypass iommu, then fill the DMAR drhd struct with + explicit device scope info. To support AMD iommu, add check of bypass iommu + when traverse the PCI hsot bridge. + - Machine and PXB options + We add bypass iommu options in machine option for default root bus, and add + option for PXB also. Note that the default value of bypass iommu is false, + so that the devices will by default go through iommu if there exist one. +