docs: add WHPX section with initial info

Signed-off-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Link: https://lore.kernel.org/r/20260327011152.4126-3-mohamed@unpredictable.fr
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

MAINTAINERS

@@ -582,6 +582,7 @@ F: include/system/whpx.h
 F: include/system/whpx-accel-ops.h
 F: include/system/whpx-common.h
 F: include/system/whpx-internal.h
+F: docs/system/whpx.rst
 
 MSHV
 M: Magnus Kulke <magnuskulke@linux.microsoft.com>

docs/system/index.rst

@@ -40,6 +40,7 @@ or Hypervisor.Framework.
    confidential-guest-support
    igvm
    nitro
+   whpx
    vm-templating
    sriov
    qemu-colo

docs/system/whpx.rst

@@ -0,0 +1,144 @@
+Windows Hypervisor Platform
+===========================
+
+Windows Hypervisor Platform is the Windows API for use of
+third-party virtual machine monitors with hardware acceleration
+on Hyper-V.
+
+It's implemented on top of ``Vid``, which is itself implemented
+on the same set of hypercalls as the ``mshv`` driver on Linux.
+
+WHPX is the name of the Windows Hypervisor Platform accelerator
+backend in QEMU. It enables using QEMU with hardware acceleration
+on both x86_64 and arm64 Windows machines.
+
+Prerequisites
+-------------
+
+WHPX requires the Windows Hypervisor Platform feature to be installed.
+
+Installation
+^^^^^^^^^^^^
+On client editions of Windows, that means installation through
+Windows Features (``optionalfeatures.exe``). On server editions,
+feature-based installation in Server Manager can be used.
+
+Alternatively, command line installation is also possible through:
+``DISM /online /Enable-Feature /FeatureName:HypervisorPlatform /All``
+
+Minimum OS version
+^^^^^^^^^^^^^^^^^^
+
+On x86_64, QEMU's Windows Hypervisor Platform backend is tested 
+starting from Windows 10 version 2004. Earlier Windows 10 releases
+*might* work but are not tested.
+
+On arm64, Windows 11 24H2 with the April 2025 optional updates
+or May 2025 security updates is the minimum required release. 
+
+Prior releases of Windows 11 version 24H2 on ARM64 shipped 
+with a pre-release version of the Windows Hypervisor Platform
+API, which is not supported in QEMU.
+
+Quick Start
+-----------
+
+Launching a virtual machine on x86_64 with WHPX acceleration::
+
+    $ qemu-system-x86_64.exe -accel whpx -M pc \
+        -smp cores=2 -m 2G -device ich9-usb-ehci1 \
+        -device usb-tablet -hda OS.qcow2
+
+Launching a virtual machine on arm64 with WHPX acceleration::
+
+    $ qemu-system-aarch64.exe -accel whpx -M virt \
+        -cpu host -smp cores=2 -m 2G \
+        -bios edk2-aarch64-code.fd \
+        -device ramfb -device nec-usb-xhci \
+        -device usb-kbd -device usb-tablet \
+        -hda OS.qcow2
+
+On arm64, for non-Windows guests, ``-device virtio-gpu-pci`` provides
+additional functionality compared to ``-device ramfb``, but is
+incompatible with Windows's UEFI GOP implementation, which
+expects a linear framebuffer to be available.
+
+Some tracing options
+--------------------
+
+x86_64
+^^^^^^
+
+``-trace whpx_unsupported_msr_access`` can be used to log accesses
+to undocumented MSRs.
+
+``-d invalid_mem`` allows to trace accesses to unmapped
+GPAs.
+
+Known issues on x86_64
+----------------------
+
+Guests using legacy VGA modes
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In guests using VGA modes that QEMU doesn't pass through framebuffer
+memory for, performance will be quite suboptimal.
+
+Workaround: for affected guests, use a more modern graphics mode.
+Alternatively, use TCG to run those guests.
+
+Guests using MMX, SSE or AVX instructions for MMIO
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Currently, ``target/i386/emulate`` does not support guests that use
+MMX, SSE or AVX instructions for access to MMIO memory ranges.
+
+Attempts to run such guests will result in an ``Unimplemented handler``
+warning for MMX and a failure to decode for newer instructions.
+
+``-M isapc``
+^^^^^^^^^^^^
+
+``-M isapc`` doesn't disable the Hyper-V LAPIC on its own yet. To
+be able to use that machine, use ``-accel whpx,hyperv=off,kernel-irqchip=off``.
+
+However, in QEMU 11.0, the guest will still be a 64-bit x86
+ISA machine with all the corresponding CPUID leaves exposed.
+
+gdbstub
+^^^^^^^
+
+As save/restore of xsave state is not currently present, state
+exposed through GDB will be incomplete.
+
+The same also applies to ``info registers``.
+
+``-cpu type`` ignored
+^^^^^^^^^^^^^^^^^^^^^
+
+In this release, -cpu is an ignored argument. 
+
+PIC interrupts on Windows 10
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+On Windows 10, a legacy PIC interrupt injected does not wake the guest
+from an HLT when using the Hyper-V provided interrupt controller.
+
+This has been addressed in QEMU 11.0 on Windows 11 platforms but
+functionality to make it available on Windows 10 isn't present.
+
+Workaround: for affected use cases, use ``-M kernel-irqchip=off``.
+
+Known issues on Windows 11
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Nested virtualisation-specific Hyper-V enlightenments are not
+currently exposed.
+
+arm64
+-----
+
+ISA feature support
+^^^^^^^^^^^^^^^^^^^
+
+SVE and SME are not currently supported.