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
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
53 --------------
55 between the VSP and the VSC.  Channels are bi-directional and used
56 for passing messages.   Most synthetic devices use a single channel,
64 The "in" ring buffer is for messages from the Hyper-V host to the
65 guest, and the "out" ring buffer is for messages from the guest to
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
105 VMbus messages
106 --------------
107 All VMbus messages have a standard header that includes the message
112 Messages follow one of two patterns:
120 responses.  Some synthetic devices allow multiple requests to be in-
122 sending a request.  Hyper-V sends back the same transactionID in the
125 Messages passed between the VSP and VSC are control messages.  For
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
133 ring buffer is minimal.  For example, time sync messages from the
134 Hyper-V host to the guest contain the actual time value.  When the
140 Three functions exist to send VMbus 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
155 fully validate messages.  With the introduction of processor
157 guest to not trust the hypervisor (AMD SNP-SEV, 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
161 messages from VMbus devices.  To facilitate such validation,
162 messages read by the guest from the "in" ring buffer are copied to a
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 ----------------
171 the guest has queued new messages in a ring buffer.  The host
173 ring buffer transitions from empty to non-empty.  If the guest sends
177 execution for a few seconds to prevent a denial-of-service attack.
181 buffer transitions from empty to non-empty.  Each CPU in the guest
182 may receive VMbus interrupts, so they are best modeled as per-CPU
184 per-CPU IRQ is allocated for VMbus.  Since x86/x64 lacks support for
185 per-CPU IRQs, an x86 interrupt vector is statically allocated (see
214 When running on later versions of Hyper-V, the CPU can be changed
238 CPU-based exclusion for correctness.  In normal operation, Hyper-V
241 when Hyper-V will make the transition.  The code must work correctly
242 even if there is a time lag before Hyper-V starts interrupting the
246 ------------------------------
247 Hyper-V and the Linux guest have a separate message-passing path
253 Hyper-V VMbus mechanism.  As part of establishing this connection,
254 the guest and Hyper-V agree on a VMbus protocol version they will
256 Hyper-V versions, and vice versa.
258 The guest then tells Hyper-V to "send offers".  Hyper-V sends an
262 identified by a GUID. The offer message from Hyper-V contains
265 two synthetic NICs will get two offers messages with the NIC
266 class ID. The ordering of offer messages can vary from boot-to-boot
268 messages may also arrive long after Linux has initially booted
269 because Hyper-V supports adding devices, such as synthetic NICs,
280 ring buffers and shares the ring buffer with the Hyper-V host by
285 setup messages via the primary channel.  These messages may include
287 VSC and the VSP on the Hyper-V host.  The setup messages may also
289 mis-named as "sub-channels" since they are functionally
295 The Hyper-V host can send a "rescind" message to the guest to
300 rescinded, neither Hyper-V nor Linux retains any state about
301 its previous existence. Such a device might be re-added later,