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 .. SPDX-License-Identifier: GPL-2.0
 
 Overview
 ========
 The Linux kernel contains a variety of code for running as a fully
 enlightened guest on Microsoft's Hyper-V hypervisor.  Hyper-V
 consists primarily of a bare-metal hypervisor plus a virtual machine
 management service running in the parent partition (roughly
 equivalent to KVM and QEMU, for example).  Guest VMs run in child
 partitions.  In this documentation, references to Hyper-V usually
 encompass both the hypervisor and the VMM service without making a
 distinction about which functionality is provided by which
 component.
 
 Hyper-V runs on x86/x64 and arm64 architectures, and Linux guests
 are supported on both.  The functionality and behavior of Hyper-V is
 generally the same on both architectures unless noted otherwise.
 
 Linux Guest Communication with Hyper-V
 --------------------------------------
 Linux guests communicate with Hyper-V in four different ways:
 
 * Implicit traps: As defined by the x86/x64 or arm64 architecture,
   some guest actions trap to Hyper-V.  Hyper-V emulates the action and
   returns control to the guest.  This behavior is generally invisible
   to the Linux kernel.
 
 * Explicit hypercalls: Linux makes an explicit function call to
   Hyper-V, passing parameters.  Hyper-V performs the requested action
   and returns control to the caller.  Parameters are passed in
   processor registers or in memory shared between the Linux guest and
   Hyper-V.   On x86/x64, hypercalls use a Hyper-V specific calling
   sequence.  On arm64, hypercalls use the ARM standard SMCCC calling
   sequence.
 
 * Synthetic register access: Hyper-V implements a variety of
   synthetic registers.  On x86/x64 these registers appear as MSRs in
   the guest, and the Linux kernel can read or write these MSRs using
   the normal mechanisms defined by the x86/x64 architecture.  On
   arm64, these synthetic registers must be accessed using explicit
   hypercalls.
 
 * VMbus: VMbus is a higher-level software construct that is built on
   the other 3 mechanisms.  It is a message passing interface between
   the Hyper-V host and the Linux guest.  It uses memory that is shared
   between Hyper-V and the guest, along with various signaling
   mechanisms.
 
 The first three communication mechanisms are documented in the
 `Hyper-V Top Level Functional Spec (TLFS)`_.  The TLFS describes
 general Hyper-V functionality and provides details on the hypercalls
 and synthetic registers.  The TLFS is currently written for the
 x86/x64 architecture only.
 
 .. _Hyper-V Top Level Functional Spec (TLFS): https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/tlfs
 
 VMbus is not documented.  This documentation provides a high-level
 overview of VMbus and how it works, but the details can be discerned
 only from the code.
 
 Sharing Memory
 --------------
 Many aspects are communication between Hyper-V and Linux are based
 on sharing memory.  Such sharing is generally accomplished as
 follows:
 
 * Linux allocates memory from its physical address space using
   standard Linux mechanisms.
 
 * Linux tells Hyper-V the guest physical address (GPA) of the
   allocated memory.  Many shared areas are kept to 1 page so that a
   single GPA is sufficient.   Larger shared areas require a list of
   GPAs, which usually do not need to be contiguous in the guest
   physical address space.  How Hyper-V is told about the GPA or list
   of GPAs varies.  In some cases, a single GPA is written to a
   synthetic register.  In other cases, a GPA or list of GPAs is sent
   in a VMbus message.
 
 * Hyper-V translates the GPAs into "real" physical memory addresses,
   and creates a virtual mapping that it can use to access the memory.
 
 * Linux can later revoke sharing it has previously established by
   telling Hyper-V to set the shared GPA to zero.
 
 Hyper-V operates with a page size of 4 Kbytes. GPAs communicated to
 Hyper-V may be in the form of page numbers, and always describe a
 range of 4 Kbytes.  Since the Linux guest page size on x86/x64 is
 also 4 Kbytes, the mapping from guest page to Hyper-V page is 1-to-1.
 On arm64, Hyper-V supports guests with 4/16/64 Kbyte pages as
 defined by the arm64 architecture.   If Linux is using 16 or 64
 Kbyte pages, Linux code must be careful to communicate with Hyper-V
 only in terms of 4 Kbyte pages.  HV_HYP_PAGE_SIZE and related macros
 are used in code that communicates with Hyper-V so that it works
 correctly in all configurations.
 
 As described in the TLFS, a few memory pages shared between Hyper-V
 and the Linux guest are "overlay" pages.  With overlay pages, Linux
 uses the usual approach of allocating guest memory and telling
 Hyper-V the GPA of the allocated memory.  But Hyper-V then replaces
 that physical memory page with a page it has allocated, and the
 original physical memory page is no longer accessible in the guest
 VM.  Linux may access the memory normally as if it were the memory
 that it originally allocated.  The "overlay" behavior is visible
 only because the contents of the page (as seen by Linux) change at
 the time that Linux originally establishes the sharing and the
 overlay page is inserted.  Similarly, the contents change if Linux
 revokes the sharing, in which case Hyper-V removes the overlay page,
 and the guest page originally allocated by Linux becomes visible
 again.
 
 Before Linux does a kexec to a kdump kernel or any other kernel,
 memory shared with Hyper-V should be revoked.  Hyper-V could modify
 a shared page or remove an overlay page after the new kernel is
 using the page for a different purpose, corrupting the new kernel.
 Hyper-V does not provide a single "set everything" operation to
 guest VMs, so Linux code must individually revoke all sharing before
 doing kexec.   See hv_kexec_handler() and hv_crash_handler().  But
 the crash/panic path still has holes in cleanup because some shared
 pages are set using per-CPU synthetic registers and there's no
 mechanism to revoke the shared pages for CPUs other than the CPU
 running the panic path.
 
 CPU Management
 --------------
 Hyper-V does not have a ability to hot-add or hot-remove a CPU
 from a running VM.  However, Windows Server 2019 Hyper-V and
 earlier versions may provide guests with ACPI tables that indicate
 more CPUs than are actually present in the VM.  As is normal, Linux
 treats these additional CPUs as potential hot-add CPUs, and reports
 them as such even though Hyper-V will never actually hot-add them.
 Starting in Windows Server 2022 Hyper-V, the ACPI tables reflect
 only the CPUs actually present in the VM, so Linux does not report
 any hot-add CPUs.
 
 A Linux guest CPU may be taken offline using the normal Linux
 mechanisms, provided no VMbus channel interrupts are assigned to
 the CPU.  See the section on VMbus Interrupts for more details
 on how VMbus channel interrupts can be re-assigned to permit
 taking a CPU offline.
 
 32-bit and 64-bit
 -----------------
 On x86/x64, Hyper-V supports 32-bit and 64-bit guests, and Linux
 will build and run in either version. While the 32-bit version is
 expected to work, it is used rarely and may suffer from undetected
 regressions.
 
 On arm64, Hyper-V supports only 64-bit guests.
 
 Endian-ness
 -----------
 All communication between Hyper-V and guest VMs uses Little-Endian
 format on both x86/x64 and arm64.  Big-endian format on arm64 is not
 supported by Hyper-V, and Linux code does not use endian-ness macros
 when accessing data shared with Hyper-V.
 
 Versioning
 ----------
 Current Linux kernels operate correctly with older versions of
 Hyper-V back to Windows Server 2012 Hyper-V. Support for running
 on the original Hyper-V release in Windows Server 2008/2008 R2
 has been removed.
 
 A Linux guest on Hyper-V outputs in dmesg the version of Hyper-V
 it is running on.  This version is in the form of a Windows build
 number and is for display purposes only. Linux code does not
 test this version number at runtime to determine available features
 and functionality. Hyper-V indicates feature/function availability
 via flags in synthetic MSRs that Hyper-V provides to the guest,
 and the guest code tests these flags.
 
 VMbus has its own protocol version that is negotiated during the
 initial VMbus connection from the guest to Hyper-V. This version
 number is also output to dmesg during boot.  This version number
 is checked in a few places in the code to determine if specific
 functionality is present.
 
 Furthermore, each synthetic device on VMbus also has a protocol
 version that is separate from the VMbus protocol version. Device
 drivers for these synthetic devices typically negotiate the device
 protocol version, and may test that protocol version to determine
 if specific device functionality is present.
 
 Code Packaging
 --------------
 Hyper-V related code appears in the Linux kernel code tree in three
 main areas:
 
 1. drivers/hv
 
 2. arch/x86/hyperv and arch/arm64/hyperv
 
 3. individual device driver areas such as drivers/scsi, drivers/net,
    drivers/clocksource, etc.
 
 A few miscellaneous files appear elsewhere. See the full list under
 "Hyper-V/Azure CORE AND DRIVERS" and "DRM DRIVER FOR HYPERV
 SYNTHETIC VIDEO DEVICE" in the MAINTAINERS file.
 
 The code in #1 and #2 is built only when CONFIG_HYPERV is set.
 Similarly, the code for most Hyper-V related drivers is built only
 when CONFIG_HYPERV is set.
 
 Most Hyper-V related code in #1 and #3 can be built as a module.
 The architecture specific code in #2 must be built-in.  Also,
 drivers/hv/hv_common.c is low-level code that is common across
 architectures and must be built-in.

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