virt/hyperv/vmbus.rst

1 .. SPDX-License-Identifier: GPL-2.0
5 VMBus is a software construct provided by Hyper-V to guest VMs.  It
7 devices that Hyper-V presents to guest VMs.   The control path is
11 and the synthetic device implementation that is part of Hyper-V, and
12 signaling primitives to allow Hyper-V and the guest to interrupt
17 establishes the VMBus control path with the Hyper-V host, then
21 Most synthetic devices offered by Hyper-V have a corresponding Linux
29 * PCI device pass-thru
34 * Key/Value Pair (KVP) exchange with Hyper-V
35 * Hyper-V online backup (a.k.a. VSS)
38 controller, synthetic NIC, and PCI pass-thru devices.  Other
39 synthetic devices are limited to a single instance per VM.  Not
41 Hyper-V that are used only by Windows guests and for which Linux
42 does not have a driver.
44 Hyper-V uses the terms "VSP" and "VSC" in describing synthetic
45 devices.  "VSP" refers to the Hyper-V code that implements a
53 --------------
55 between the VSP and the VSC.  Channels are bi-directional and used
63 empty, so a full ring buffer always has at least one byte unused.
64 The "in" ring buffer is for messages from the Hyper-V host to the
66 the Hyper-V host.  In Linux, the "in" and "out" designations are as
75 making up the ring is communicated to the Hyper-V host over the
84 ring buffer need not be concerned with ring buffer wrap-around.
87 actual data copy does not need to be broken into two parts.  This
89 directly in the ring without handling wrap-around.
92 passed to Hyper-V as a 4 Kbyte area.  But the memory for the actual
95 a portion of the header page is unused and not communicated to
96 Hyper-V.  This case is handled by vmbus_establish_gpadl().
98 Hyper-V enforces a limit on the aggregate amount of guest memory
106 ----------------------
114 * Unidirectional:  Either side sends a message and does not
120 responses.  Some synthetic devices allow multiple requests to be in-
122 sending a request.  Hyper-V sends back the same transactionID in the
128 between the guest and the Hyper-V host, the actual data to be
130 specified as a separate data buffer that the Hyper-V host will
134 Hyper-V host to the guest contain the actual time value.  When the
140 Three functions exist to send VMBus channel messages:
142 1. vmbus_sendpacket():  Control-only messages and messages with
143    embedded data -- no GPAs
153 Historically, Linux guests have trusted Hyper-V to send well-formed
154 and valid messages, and Linux drivers for synthetic devices did not
157 guest to not trust the hypervisor (AMD SEV-SNP, Intel TDX), trusting
158 the Hyper-V host is no longer a valid assumption.  The drivers for
160 values read from memory that is shared with Hyper-V, which includes
163 temporary buffer that is not shared with Hyper-V.  Validation is
164 performed in this temporary buffer without the risk of Hyper-V
169 --------------------------------------
170 Hyper-V provides each guest CPU with a synthetic interrupt controller
171 that is used by VMBus for host-guest communication. While each synic
174 the Hyper-V host and a guest CPU use that SINT.
176 The SINT is mapped to a single per-CPU architectural interrupt (i.e,
177 an 8-bit x86/x64 interrupt vector, or an arm64 PPI INTID). Because
179 they are best modeled in Linux as per-CPU interrupts. This model works
180 well on arm64 where a single per-CPU Linux IRQ is allocated for
182 "Hyper-V VMbus". Since x86/x64 lacks support for per-CPU IRQs, an x86
193 The synic is not modeled in Linux as an irq chip or irq domain,
194 and the demultiplexed logical interrupts are not Linux IRQs. As such,
200 ----------------
203 expects that the guest will send an interrupt only when an "out"
204 ring buffer transitions from empty to non-empty.  If the guest sends
208 execution for a few seconds to prevent a denial-of-service attack.
212 channel "in" ring buffer transitions from empty to non-empty due to
247 When running on later versions of Hyper-V, the CPU can be changed
249 interrupts are not Linux IRQs, there are no entries in /proc/interrupts
252 An online CPU in a Linux guest may not be taken offline if it has
260 CPU assigned to the channel.  Specifically, the code does not use
261 CPU-based exclusion for correctness.  In normal operation, Hyper-V
264 when Hyper-V will make the transition.  The code must work correctly
265 even if there is a time lag before Hyper-V starts interrupting the
269 ------------------------------
270 Hyper-V and the Linux guest have a separate message-passing path
272 path does not use a VMBus channel.  See vmbus_post_msg() and
276 Hyper-V VMBus mechanism.  As part of establishing this connection,
277 the guest and Hyper-V agree on a VMBus protocol version they will
279 Hyper-V versions, and vice versa.
281 The guest then tells Hyper-V to "send offers".  Hyper-V sends an
285 identified by a GUID. The offer message from Hyper-V contains
289 class ID. The ordering of offer messages can vary from boot-to-boot
290 and must not be assumed to be consistent in Linux code. Offer
292 because Hyper-V supports adding devices, such as synthetic NICs,
303 ring buffers and shares the ring buffer with the Hyper-V host by
310 VSC and the VSP on the Hyper-V host.  The setup messages may also
312 mis-named as "sub-channels" since they are functionally
318 The Hyper-V host can send a "rescind" message to the guest to
323 rescinded, neither Hyper-V nor Linux retains any state about
324 its previous existence. Such a device might be re-added later,