Merge branch 'xenfv-kvm-15' of git://git.infradead.org/users/dwmw2/qemu into HEAD

This adds support for emulating Xen under Linux/KVM, based on kernel
patches which have been present since Linux v5.12. As with the kernel
support, it's derived from work started by João Martins of Oracle in
2018.

This series just adds the basic platform support — CPUID, hypercalls,
event channels, a stub of XenStore.

A full single-tenant internal implementation of XenStore, and patches
to make QEMU's Xen PV drivers work with this Xen emulation, are waiting
in the wings to be submitted in a follow-on patch series.

As noted in the documentation, it's enabled by setting the xen-version
property on the KVM accelerator, e.g.:

 qemu-system-x86_64 -serial mon:stdio -M q35 -display none -m 1G -smp 2 \
    -accel kvm,xen-version=0x4000e,kernel-irqchip=split \
    -kernel vmlinuz-6.0.7-301.fc37.x86_64 \
    -append "console=ttyS0 root=/dev/sda1" \
    -drive file=/var/lib/libvirt/images/fedora28.qcow2,if=none,id=disk \
    -device ahci,id=ahci -device ide-hd,drive=disk,bus=ahci.0

Even before this was merged, we've already been using it to find and fix
bugs in the Linux kernel Xen guest support:

https://lore.kernel.org/all/4bffa69a949bfdc92c4a18e5a1c3cbb3b94a0d32.camel@infradead.org/
https://lore.kernel.org/all/871qnunycr.ffs@tglx/

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 77 insertions, 0 deletions

diff --git a/docs/system/i386/xen.rst b/docs/system/i386/xen.rst
new file mode 100644
index 0000000000..a00523b492
--- /dev/null
+++ b/docs/system/i386/xen.rst
@@ -0,0 +1,76 @@
+Xen HVM guest support
+=====================
+
+
+Description
+-----------
+
+KVM has support for hosting Xen guests, intercepting Xen hypercalls and event
+channel (Xen PV interrupt) delivery. This allows guests which expect to be
+run under Xen to be hosted in QEMU under Linux/KVM instead.
+
+Setup
+-----
+
+Xen mode is enabled by setting the ``xen-version`` property of the KVM
+accelerator, for example for Xen 4.10:
+
+.. parsed-literal::
+
+  |qemu_system| --accel kvm,xen-version=0x4000a
+
+Additionally, virtual APIC support can be advertised to the guest through the
+``xen-vapic`` CPU flag:
+
+.. parsed-literal::
+
+  |qemu_system| --accel kvm,xen-version=0x4000a --cpu host,+xen_vapic
+
+When Xen support is enabled, QEMU changes hypervisor identification (CPUID
+0x40000000..0x4000000A) to Xen. The KVM identification and features are not
+advertised to a Xen guest. If Hyper-V is also enabled, the Xen identification
+moves to leaves 0x40000100..0x4000010A.
+
+The Xen platform device is enabled automatically for a Xen guest. This allows
+a guest to unplug all emulated devices, in order to use Xen PV block and network
+drivers instead. Note that until the Xen PV device back ends are enabled to work
+with Xen mode in QEMU, that is unlikely to cause significant joy. Linux guests
+can be dissuaded from this by adding 'xen_emul_unplug=never' on their command
+line, and it can also be noted that AHCI disk controllers are exempt from being
+unplugged, as are passthrough VFIO PCI devices.
+
+Properties
+----------
+
+The following properties exist on the KVM accelerator object:
+
+``xen-version``
+  This property contains the Xen version in ``XENVER_version`` form, with the
+  major version in the top 16 bits and the minor version in the low 16 bits.
+  Setting this property enables the Xen guest support.
+
+``xen-evtchn-max-pirq``
+  Xen PIRQs represent an emulated physical interrupt, either GSI or MSI, which
+  can be routed to an event channel instead of to the emulated I/O or local
+  APIC. By default, QEMU permits only 256 PIRQs because this allows maximum
+  compatibility with 32-bit MSI where the higher bits of the PIRQ# would need
+  to be in the upper 64 bits of the MSI message. For guests with large numbers
+  of PCI devices (and none which are limited to 32-bit addressing) it may be
+  desirable to increase this value.
+
+``xen-gnttab-max-frames``
+  Xen grant tables are the means by which a Xen guest grants access to its
+  memory for PV back ends (disk, network, etc.). Since QEMU only supports v1
+  grant tables which are 8 bytes in size, each page (each frame) of the grant
+  table can reference 512 pages of guest memory. The default number of frames
+  is 64, allowing for 32768 pages of guest memory to be accessed by PV backends
+  through simultaneous grants. For guests with large numbers of PV devices and
+  high throughput, it may be desirable to increase this value.
+
+OS requirements
+---------------
+
+The minimal Xen support in the KVM accelerator requires the host to be running
+Linux v5.12 or newer. Later versions add optimisations: Linux v5.17 added
+acceleration of interrupt delivery via the Xen PIRQ mechanism, and Linux v5.19
+accelerated Xen PV timers and inter-processor interrupts (IPIs).
diff --git a/docs/system/target-i386.rst b/docs/system/target-i386.rst
index e64c013077..77c2f3b979 100644
--- a/docs/system/target-i386.rst
+++ b/docs/system/target-i386.rst
@@ -27,6 +27,7 @@ Architectural features
 
    i386/cpu
    i386/hyperv
+   i386/xen
    i386/kvm-pv
    i386/sgx
    i386/amd-memory-encryption