docs/system/i386: xenpvh: Add a basic description

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

MAINTAINERS

@@ -560,6 +560,7 @@ F: include/sysemu/xen.h
 F: include/sysemu/xen-mapcache.h
 F: stubs/xen-hw-stub.c
 F: docs/system/arm/xenpvh.rst
+F: docs/system/i386/xenpvh.rst
 
 Guest CPU Cores (NVMM)
 ----------------------

docs/system/i386/xenpvh.rst

@@ -0,0 +1,49 @@
+Xen PVH machine (``xenpvh``)
+=========================================
+
+Xen supports a spectrum of types of guests that vary in how they depend
+on HW virtualization features, emulation models and paravirtualization.
+PVH is a mode that uses HW virtualization features (like HVM) but tries
+to avoid emulation models and instead use passthrough or
+paravirtualized devices.
+
+QEMU can be used to provide PV virtio devices on an emulated PCIe controller.
+That is the purpose of this minimal machine.
+
+Supported devices
+-----------------
+
+The x86 Xen PVH QEMU machine provide the following devices:
+
+- RAM
+- GPEX host bridge
+- virtio-pci devices
+
+The idea is to only connect virtio-pci devices but in theory any compatible
+PCI device model will work depending on Xen and guest support.
+
+Running
+-------
+
+The Xen tools will typically construct a command-line and launch QEMU
+for you when needed. But here's an example of what it can look like in
+case you need to construct one manually:
+
+.. code-block:: console
+
+    qemu-system-i386 -xen-domid 3 -no-shutdown        \
+      -chardev socket,id=libxl-cmd,path=/var/run/xen/qmp-libxl-3,server=on,wait=off \
+      -mon chardev=libxl-cmd,mode=control             \
+      -chardev socket,id=libxenstat-cmd,path=/var/run/xen/qmp-libxenstat-3,server=on,wait=off \
+      -mon chardev=libxenstat-cmd,mode=control        \
+      -nodefaults                                     \
+      -no-user-config                                 \
+      -xen-attach -name g0                            \
+      -vnc none                                       \
+      -display none                                   \
+      -device virtio-net-pci,id=nic0,netdev=net0,mac=00:16:3e:5c:81:78 \
+      -netdev type=tap,id=net0,ifname=vif3.0-emu,br=xenbr0,script=no,downscript=no \
+      -smp 4,maxcpus=4                                \
+      -nographic                                      \
+      -machine xenpvh,ram-low-base=0,ram-low-size=2147483648,ram-high-base=4294967296,ram-high-size=2147483648,pci-ecam-base=824633720832,pci-ecam-size=268435456,pci-mmio-base=4026531840,pci-mmio-size=33554432,pci-mmio-high-base=824902156288,pci-mmio-high-size=68719476736 \
+      -m 4096

docs/system/target-i386.rst

@@ -26,6 +26,7 @@ Architectural features
    i386/cpu
    i386/hyperv
    i386/xen
+   i386/xenpvh
    i386/kvm-pv
    i386/sgx
    i386/amd-memory-encryption