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diff --git a/Documentation/virt/hyperv/vmbus.rst b/Documentation/virt/hyperv/vmbus.rst new file mode 100644 index 000000000..d2012d902 --- /dev/null +++ b/Documentation/virt/hyperv/vmbus.rst @@ -0,0 +1,303 @@ +.. SPDX-License-Identifier: GPL-2.0 + +VMbus +===== +VMbus is a software construct provided by Hyper-V to guest VMs. It +consists of a control path and common facilities used by synthetic +devices that Hyper-V presents to guest VMs. The control path is +used to offer synthetic devices to the guest VM and, in some cases, +to rescind those devices. The common facilities include software +channels for communicating between the device driver in the guest VM +and the synthetic device implementation that is part of Hyper-V, and +signaling primitives to allow Hyper-V and the guest to interrupt +each other. + +VMbus is modeled in Linux as a bus, with the expected /sys/bus/vmbus +entry in a running Linux guest. The VMbus driver (drivers/hv/vmbus_drv.c) +establishes the VMbus control path with the Hyper-V host, then +registers itself as a Linux bus driver. It implements the standard +bus functions for adding and removing devices to/from the bus. + +Most synthetic devices offered by Hyper-V have a corresponding Linux +device driver. These devices include: + +* SCSI controller +* NIC +* Graphics frame buffer +* Keyboard +* Mouse +* PCI device pass-thru +* Heartbeat +* Time Sync +* Shutdown +* Memory balloon +* Key/Value Pair (KVP) exchange with Hyper-V +* Hyper-V online backup (a.k.a. VSS) + +Guest VMs may have multiple instances of the synthetic SCSI +controller, synthetic NIC, and PCI pass-thru devices. Other +synthetic devices are limited to a single instance per VM. Not +listed above are a small number of synthetic devices offered by +Hyper-V that are used only by Windows guests and for which Linux +does not have a driver. + +Hyper-V uses the terms "VSP" and "VSC" in describing synthetic +devices. "VSP" refers to the Hyper-V code that implements a +particular synthetic device, while "VSC" refers to the driver for +the device in the guest VM. For example, the Linux driver for the +synthetic NIC is referred to as "netvsc" and the Linux driver for +the synthetic SCSI controller is "storvsc". These drivers contain +functions with names like "storvsc_connect_to_vsp". + +VMbus channels +-------------- +An instance of a synthetic device uses VMbus channels to communicate +between the VSP and the VSC. Channels are bi-directional and used +for passing messages. Most synthetic devices use a single channel, +but the synthetic SCSI controller and synthetic NIC may use multiple +channels to achieve higher performance and greater parallelism. + +Each channel consists of two ring buffers. These are classic ring +buffers from a university data structures textbook. If the read +and writes pointers are equal, the ring buffer is considered to be +empty, so a full ring buffer always has at least one byte unused. +The "in" ring buffer is for messages from the Hyper-V host to the +guest, and the "out" ring buffer is for messages from the guest to +the Hyper-V host. In Linux, the "in" and "out" designations are as +viewed by the guest side. The ring buffers are memory that is +shared between the guest and the host, and they follow the standard +paradigm where the memory is allocated by the guest, with the list +of GPAs that make up the ring buffer communicated to the host. Each +ring buffer consists of a header page (4 Kbytes) with the read and +write indices and some control flags, followed by the memory for the +actual ring. The size of the ring is determined by the VSC in the +guest and is specific to each synthetic device. The list of GPAs +making up the ring is communicated to the Hyper-V host over the +VMbus control path as a GPA Descriptor List (GPADL). See function +vmbus_establish_gpadl(). + +Each ring buffer is mapped into contiguous Linux kernel virtual +space in three parts: 1) the 4 Kbyte header page, 2) the memory +that makes up the ring itself, and 3) a second mapping of the memory +that makes up the ring itself. Because (2) and (3) are contiguous +in kernel virtual space, the code that copies data to and from the +ring buffer need not be concerned with ring buffer wrap-around. +Once a copy operation has completed, the read or write index may +need to be reset to point back into the first mapping, but the +actual data copy does not need to be broken into two parts. This +approach also allows complex data structures to be easily accessed +directly in the ring without handling wrap-around. + +On arm64 with page sizes > 4 Kbytes, the header page must still be +passed to Hyper-V as a 4 Kbyte area. But the memory for the actual +ring must be aligned to PAGE_SIZE and have a size that is a multiple +of PAGE_SIZE so that the duplicate mapping trick can be done. Hence +a portion of the header page is unused and not communicated to +Hyper-V. This case is handled by vmbus_establish_gpadl(). + +Hyper-V enforces a limit on the aggregate amount of guest memory +that can be shared with the host via GPADLs. This limit ensures +that a rogue guest can't force the consumption of excessive host +resources. For Windows Server 2019 and later, this limit is +approximately 1280 Mbytes. For versions prior to Windows Server +2019, the limit is approximately 384 Mbytes. + +VMbus messages +-------------- +All VMbus messages have a standard header that includes the message +length, the offset of the message payload, some flags, and a +transactionID. The portion of the message after the header is +unique to each VSP/VSC pair. + +Messages follow one of two patterns: + +* Unidirectional: Either side sends a message and does not + expect a response message +* Request/response: One side (usually the guest) sends a message + and expects a response + +The transactionID (a.k.a. "requestID") is for matching requests & +responses. Some synthetic devices allow multiple requests to be in- +flight simultaneously, so the guest specifies a transactionID when +sending a request. Hyper-V sends back the same transactionID in the +matching response. + +Messages passed between the VSP and VSC are control messages. For +example, a message sent from the storvsc driver might be "execute +this SCSI command". If a message also implies some data transfer +between the guest and the Hyper-V host, the actual data to be +transferred may be embedded with the control message, or it may be +specified as a separate data buffer that the Hyper-V host will +access as a DMA operation. The former case is used when the size of +the data is small and the cost of copying the data to and from the +ring buffer is minimal. For example, time sync messages from the +Hyper-V host to the guest contain the actual time value. When the +data is larger, a separate data buffer is used. In this case, the +control message contains a list of GPAs that describe the data +buffer. For example, the storvsc driver uses this approach to +specify the data buffers to/from which disk I/O is done. + +Three functions exist to send VMbus messages: + +1. vmbus_sendpacket(): Control-only messages and messages with + embedded data -- no GPAs +2. vmbus_sendpacket_pagebuffer(): Message with list of GPAs + identifying data to transfer. An offset and length is + associated with each GPA so that multiple discontinuous areas + of guest memory can be targeted. +3. vmbus_sendpacket_mpb_desc(): Message with list of GPAs + identifying data to transfer. A single offset and length is + associated with a list of GPAs. The GPAs must describe a + single logical area of guest memory to be targeted. + +Historically, Linux guests have trusted Hyper-V to send well-formed +and valid messages, and Linux drivers for synthetic devices did not +fully validate messages. With the introduction of processor +technologies that fully encrypt guest memory and that allow the +guest to not trust the hypervisor (AMD SNP-SEV, Intel TDX), trusting +the Hyper-V host is no longer a valid assumption. The drivers for +VMbus synthetic devices are being updated to fully validate any +values read from memory that is shared with Hyper-V, which includes +messages from VMbus devices. To facilitate such validation, +messages read by the guest from the "in" ring buffer are copied to a +temporary buffer that is not shared with Hyper-V. Validation is +performed in this temporary buffer without the risk of Hyper-V +maliciously modifying the message after it is validated but before +it is used. + +VMbus interrupts +---------------- +VMbus provides a mechanism for the guest to interrupt the host when +the guest has queued new messages in a ring buffer. The host +expects that the guest will send an interrupt only when an "out" +ring buffer transitions from empty to non-empty. If the guest sends +interrupts at other times, the host deems such interrupts to be +unnecessary. If a guest sends an excessive number of unnecessary +interrupts, the host may throttle that guest by suspending its +execution for a few seconds to prevent a denial-of-service attack. + +Similarly, the host will interrupt the guest when it sends a new +message on the VMbus control path, or when a VMbus channel "in" ring +buffer transitions from empty to non-empty. Each CPU in the guest +may receive VMbus interrupts, so they are best modeled as per-CPU +interrupts in Linux. This model works well on arm64 where a single +per-CPU IRQ is allocated for VMbus. Since x86/x64 lacks support for +per-CPU IRQs, an x86 interrupt vector is statically allocated (see +HYPERVISOR_CALLBACK_VECTOR) across all CPUs and explicitly coded to +call the VMbus interrupt service routine. These interrupts are +visible in /proc/interrupts on the "HYP" line. + +The guest CPU that a VMbus channel will interrupt is selected by the +guest when the channel is created, and the host is informed of that +selection. VMbus devices are broadly grouped into two categories: + +1. "Slow" devices that need only one VMbus channel. The devices + (such as keyboard, mouse, heartbeat, and timesync) generate + relatively few interrupts. Their VMbus channels are all + assigned to interrupt the VMBUS_CONNECT_CPU, which is always + CPU 0. + +2. "High speed" devices that may use multiple VMbus channels for + higher parallelism and performance. These devices include the + synthetic SCSI controller and synthetic NIC. Their VMbus + channels interrupts are assigned to CPUs that are spread out + among the available CPUs in the VM so that interrupts on + multiple channels can be processed in parallel. + +The assignment of VMbus channel interrupts to CPUs is done in the +function init_vp_index(). This assignment is done outside of the +normal Linux interrupt affinity mechanism, so the interrupts are +neither "unmanaged" nor "managed" interrupts. + +The CPU that a VMbus channel will interrupt can be seen in +/sys/bus/vmbus/devices/<deviceGUID>/ channels/<channelRelID>/cpu. +When running on later versions of Hyper-V, the CPU can be changed +by writing a new value to this sysfs entry. Because the interrupt +assignment is done outside of the normal Linux affinity mechanism, +there are no entries in /proc/irq corresponding to individual +VMbus channel interrupts. + +An online CPU in a Linux guest may not be taken offline if it has +VMbus channel interrupts assigned to it. Any such channel +interrupts must first be manually reassigned to another CPU as +described above. When no channel interrupts are assigned to the +CPU, it can be taken offline. + +When a guest CPU receives a VMbus interrupt from the host, the +function vmbus_isr() handles the interrupt. It first checks for +channel interrupts by calling vmbus_chan_sched(), which looks at a +bitmap setup by the host to determine which channels have pending +interrupts on this CPU. If multiple channels have pending +interrupts for this CPU, they are processed sequentially. When all +channel interrupts have been processed, vmbus_isr() checks for and +processes any message received on the VMbus control path. + +The VMbus channel interrupt handling code is designed to work +correctly even if an interrupt is received on a CPU other than the +CPU assigned to the channel. Specifically, the code does not use +CPU-based exclusion for correctness. In normal operation, Hyper-V +will interrupt the assigned CPU. But when the CPU assigned to a +channel is being changed via sysfs, the guest doesn't know exactly +when Hyper-V will make the transition. The code must work correctly +even if there is a time lag before Hyper-V starts interrupting the +new CPU. See comments in target_cpu_store(). + +VMbus device creation/deletion +------------------------------ +Hyper-V and the Linux guest have a separate message-passing path +that is used for synthetic device creation and deletion. This +path does not use a VMbus channel. See vmbus_post_msg() and +vmbus_on_msg_dpc(). + +The first step is for the guest to connect to the generic +Hyper-V VMbus mechanism. As part of establishing this connection, +the guest and Hyper-V agree on a VMbus protocol version they will +use. This negotiation allows newer Linux kernels to run on older +Hyper-V versions, and vice versa. + +The guest then tells Hyper-V to "send offers". Hyper-V sends an +offer message to the guest for each synthetic device that the VM +is configured to have. Each VMbus device type has a fixed GUID +known as the "class ID", and each VMbus device instance is also +identified by a GUID. The offer message from Hyper-V contains +both GUIDs to uniquely (within the VM) identify the device. +There is one offer message for each device instance, so a VM with +two synthetic NICs will get two offers messages with the NIC +class ID. The ordering of offer messages can vary from boot-to-boot +and must not be assumed to be consistent in Linux code. Offer +messages may also arrive long after Linux has initially booted +because Hyper-V supports adding devices, such as synthetic NICs, +to running VMs. A new offer message is processed by +vmbus_process_offer(), which indirectly invokes vmbus_add_channel_work(). + +Upon receipt of an offer message, the guest identifies the device +type based on the class ID, and invokes the correct driver to set up +the device. Driver/device matching is performed using the standard +Linux mechanism. + +The device driver probe function opens the primary VMbus channel to +the corresponding VSP. It allocates guest memory for the channel +ring buffers and shares the ring buffer with the Hyper-V host by +giving the host a list of GPAs for the ring buffer memory. See +vmbus_establish_gpadl(). + +Once the ring buffer is set up, the device driver and VSP exchange +setup messages via the primary channel. These messages may include +negotiating the device protocol version to be used between the Linux +VSC and the VSP on the Hyper-V host. The setup messages may also +include creating additional VMbus channels, which are somewhat +mis-named as "sub-channels" since they are functionally +equivalent to the primary channel once they are created. + +Finally, the device driver may create entries in /dev as with +any device driver. + +The Hyper-V host can send a "rescind" message to the guest to +remove a device that was previously offered. Linux drivers must +handle such a rescind message at any time. Rescinding a device +invokes the device driver "remove" function to cleanly shut +down the device and remove it. Once a synthetic device is +rescinded, neither Hyper-V nor Linux retains any state about +its previous existence. Such a device might be re-added later, +in which case it is treated as an entirely new device. See +vmbus_onoffer_rescind(). |