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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-11 08:27:49 +0000
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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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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.