mirror of https://github.com/xemu-project/xemu.git
145 lines
5.9 KiB
ReStructuredText
145 lines
5.9 KiB
ReStructuredText
Xen HVM guest support
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=====================
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Description
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-----------
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KVM has support for hosting Xen guests, intercepting Xen hypercalls and event
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channel (Xen PV interrupt) delivery. This allows guests which expect to be
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run under Xen to be hosted in QEMU under Linux/KVM instead.
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Using the split irqchip is mandatory for Xen support.
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Setup
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-----
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Xen mode is enabled by setting the ``xen-version`` property of the KVM
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accelerator, for example for Xen 4.17:
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.. parsed-literal::
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|qemu_system| --accel kvm,xen-version=0x40011,kernel-irqchip=split
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Additionally, virtual APIC support can be advertised to the guest through the
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``xen-vapic`` CPU flag:
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.. parsed-literal::
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|qemu_system| --accel kvm,xen-version=0x40011,kernel-irqchip=split --cpu host,+xen-vapic
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When Xen support is enabled, QEMU changes hypervisor identification (CPUID
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0x40000000..0x4000000A) to Xen. The KVM identification and features are not
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advertised to a Xen guest. If Hyper-V is also enabled, the Xen identification
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moves to leaves 0x40000100..0x4000010A.
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Properties
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----------
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The following properties exist on the KVM accelerator object:
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``xen-version``
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This property contains the Xen version in ``XENVER_version`` form, with the
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major version in the top 16 bits and the minor version in the low 16 bits.
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Setting this property enables the Xen guest support. If Xen version 4.5 or
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greater is specified, the HVM leaf in Xen CPUID is populated. Xen version
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4.6 enables the vCPU ID in CPUID, and version 4.17 advertises vCPU upcall
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vector support to the guest.
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``xen-evtchn-max-pirq``
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Xen PIRQs represent an emulated physical interrupt, either GSI or MSI, which
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can be routed to an event channel instead of to the emulated I/O or local
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APIC. By default, QEMU permits only 256 PIRQs because this allows maximum
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compatibility with 32-bit MSI where the higher bits of the PIRQ# would need
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to be in the upper 64 bits of the MSI message. For guests with large numbers
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of PCI devices (and none which are limited to 32-bit addressing) it may be
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desirable to increase this value.
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``xen-gnttab-max-frames``
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Xen grant tables are the means by which a Xen guest grants access to its
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memory for PV back ends (disk, network, etc.). Since QEMU only supports v1
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grant tables which are 8 bytes in size, each page (each frame) of the grant
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table can reference 512 pages of guest memory. The default number of frames
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is 64, allowing for 32768 pages of guest memory to be accessed by PV backends
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through simultaneous grants. For guests with large numbers of PV devices and
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high throughput, it may be desirable to increase this value.
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Xen paravirtual devices
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-----------------------
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The Xen PCI platform device is enabled automatically for a Xen guest. This
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allows a guest to unplug all emulated devices, in order to use paravirtual
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block and network drivers instead.
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Those paravirtual Xen block, network (and console) devices can be created
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through the command line, and/or hot-plugged.
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To provide a Xen console device, define a character device and then a device
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of type ``xen-console`` to connect to it. For the Xen console equivalent of
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the handy ``-serial mon:stdio`` option, for example:
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.. parsed-literal::
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-chardev stdio,mux=on,id=char0,signal=off -mon char0 \\
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-device xen-console,chardev=char0
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The Xen network device is ``xen-net-device``, which becomes the default NIC
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model for emulated Xen guests, meaning that just the default NIC provided
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by QEMU should automatically work and present a Xen network device to the
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guest.
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Disks can be configured with '``-drive file=${GUEST_IMAGE},if=xen``' and will
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appear to the guest as ``xvda`` onwards.
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Under Xen, the boot disk is typically available both via IDE emulation, and
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as a PV block device. Guest bootloaders typically use IDE to load the guest
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kernel, which then unplugs the IDE and continues with the Xen PV block device.
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This configuration can be achieved as follows:
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.. parsed-literal::
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|qemu_system| --accel kvm,xen-version=0x40011,kernel-irqchip=split \\
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-drive file=${GUEST_IMAGE},if=xen \\
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-drive file=${GUEST_IMAGE},file.locking=off,if=ide
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VirtIO devices can also be used; Linux guests may need to be dissuaded from
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umplugging them by adding '``xen_emul_unplug=never``' on their command line.
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Booting Xen PV guests
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---------------------
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Booting PV guest kernels is possible by using the Xen PV shim (a version of Xen
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itself, designed to run inside a Xen HVM guest and provide memory management
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services for one guest alone).
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The Xen binary is provided as the ``-kernel`` and the guest kernel itself (or
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PV Grub image) as the ``-initrd`` image, which actually just means the first
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multiboot "module". For example:
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.. parsed-literal::
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|qemu_system| --accel kvm,xen-version=0x40011,kernel-irqchip=split \\
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-chardev stdio,id=char0 -device xen-console,chardev=char0 \\
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-display none -m 1G -kernel xen -initrd bzImage \\
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-append "pv-shim console=xen,pv -- console=hvc0 root=/dev/xvda1" \\
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-drive file=${GUEST_IMAGE},if=xen
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The Xen image must be built with the ``CONFIG_XEN_GUEST`` and ``CONFIG_PV_SHIM``
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options, and as of Xen 4.17, Xen's PV shim mode does not support using a serial
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port; it must have a Xen console or it will panic.
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The example above provides the guest kernel command line after a separator
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(" ``--`` ") on the Xen command line, and does not provide the guest kernel
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with an actual initramfs, which would need to listed as a second multiboot
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module. For more complicated alternatives, see the command line
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:ref:`documentation <system/invocation-qemu-options-initrd>` for the
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``-initrd`` option.
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Host OS requirements
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--------------------
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The minimal Xen support in the KVM accelerator requires the host to be running
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Linux v5.12 or newer. Later versions add optimisations: Linux v5.17 added
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acceleration of interrupt delivery via the Xen PIRQ mechanism, and Linux v5.19
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accelerated Xen PV timers and inter-processor interrupts (IPIs).
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