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diff --git a/Documentation/virt/hyperv/overview.rst b/Documentation/virt/hyperv/overview.rst new file mode 100644 index 0000000000..cd493332c8 --- /dev/null +++ b/Documentation/virt/hyperv/overview.rst @@ -0,0 +1,207 @@ +.. 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. |