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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
41 Hyper-V that are used only by Windows guests and for which Linux
44 Hyper-V uses the terms "VSP" and "VSC" in describing synthetic
45 devices. "VSP" refers to the Hyper-V code that implements a
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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.
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
96 Hyper-V. This case is handled by vmbus_establish_gpadl().
98 Hyper-V enforces a limit on the aggregate amount of guest memory
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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
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
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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
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
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
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,