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diff --git a/Documentation/virt/hyperv/vpci.rst b/Documentation/virt/hyperv/vpci.rst new file mode 100644 index 0000000000..b65b2126ed --- /dev/null +++ b/Documentation/virt/hyperv/vpci.rst @@ -0,0 +1,316 @@ +.. SPDX-License-Identifier: GPL-2.0 + +PCI pass-thru devices +========================= +In a Hyper-V guest VM, PCI pass-thru devices (also called +virtual PCI devices, or vPCI devices) are physical PCI devices +that are mapped directly into the VM's physical address space. +Guest device drivers can interact directly with the hardware +without intermediation by the host hypervisor. This approach +provides higher bandwidth access to the device with lower +latency, compared with devices that are virtualized by the +hypervisor. The device should appear to the guest just as it +would when running on bare metal, so no changes are required +to the Linux device drivers for the device. + +Hyper-V terminology for vPCI devices is "Discrete Device +Assignment" (DDA). Public documentation for Hyper-V DDA is +available here: `DDA`_ + +.. _DDA: https://learn.microsoft.com/en-us/windows-server/virtualization/hyper-v/plan/plan-for-deploying-devices-using-discrete-device-assignment + +DDA is typically used for storage controllers, such as NVMe, +and for GPUs. A similar mechanism for NICs is called SR-IOV +and produces the same benefits by allowing a guest device +driver to interact directly with the hardware. See Hyper-V +public documentation here: `SR-IOV`_ + +.. _SR-IOV: https://learn.microsoft.com/en-us/windows-hardware/drivers/network/overview-of-single-root-i-o-virtualization--sr-iov- + +This discussion of vPCI devices includes DDA and SR-IOV +devices. + +Device Presentation +------------------- +Hyper-V provides full PCI functionality for a vPCI device when +it is operating, so the Linux device driver for the device can +be used unchanged, provided it uses the correct Linux kernel +APIs for accessing PCI config space and for other integration +with Linux. But the initial detection of the PCI device and +its integration with the Linux PCI subsystem must use Hyper-V +specific mechanisms. Consequently, vPCI devices on Hyper-V +have a dual identity. They are initially presented to Linux +guests as VMBus devices via the standard VMBus "offer" +mechanism, so they have a VMBus identity and appear under +/sys/bus/vmbus/devices. The VMBus vPCI driver in Linux at +drivers/pci/controller/pci-hyperv.c handles a newly introduced +vPCI device by fabricating a PCI bus topology and creating all +the normal PCI device data structures in Linux that would +exist if the PCI device were discovered via ACPI on a bare- +metal system. Once those data structures are set up, the +device also has a normal PCI identity in Linux, and the normal +Linux device driver for the vPCI device can function as if it +were running in Linux on bare-metal. Because vPCI devices are +presented dynamically through the VMBus offer mechanism, they +do not appear in the Linux guest's ACPI tables. vPCI devices +may be added to a VM or removed from a VM at any time during +the life of the VM, and not just during initial boot. + +With this approach, the vPCI device is a VMBus device and a +PCI device at the same time. In response to the VMBus offer +message, the hv_pci_probe() function runs and establishes a +VMBus connection to the vPCI VSP on the Hyper-V host. That +connection has a single VMBus channel. The channel is used to +exchange messages with the vPCI VSP for the purpose of setting +up and configuring the vPCI device in Linux. Once the device +is fully configured in Linux as a PCI device, the VMBus +channel is used only if Linux changes the vCPU to be interrupted +in the guest, or if the vPCI device is removed from +the VM while the VM is running. The ongoing operation of the +device happens directly between the Linux device driver for +the device and the hardware, with VMBus and the VMBus channel +playing no role. + +PCI Device Setup +---------------- +PCI device setup follows a sequence that Hyper-V originally +created for Windows guests, and that can be ill-suited for +Linux guests due to differences in the overall structure of +the Linux PCI subsystem compared with Windows. Nonetheless, +with a bit of hackery in the Hyper-V virtual PCI driver for +Linux, the virtual PCI device is setup in Linux so that +generic Linux PCI subsystem code and the Linux driver for the +device "just work". + +Each vPCI device is set up in Linux to be in its own PCI +domain with a host bridge. The PCI domainID is derived from +bytes 4 and 5 of the instance GUID assigned to the VMBus vPCI +device. The Hyper-V host does not guarantee that these bytes +are unique, so hv_pci_probe() has an algorithm to resolve +collisions. The collision resolution is intended to be stable +across reboots of the same VM so that the PCI domainIDs don't +change, as the domainID appears in the user space +configuration of some devices. + +hv_pci_probe() allocates a guest MMIO range to be used as PCI +config space for the device. This MMIO range is communicated +to the Hyper-V host over the VMBus channel as part of telling +the host that the device is ready to enter d0. See +hv_pci_enter_d0(). When the guest subsequently accesses this +MMIO range, the Hyper-V host intercepts the accesses and maps +them to the physical device PCI config space. + +hv_pci_probe() also gets BAR information for the device from +the Hyper-V host, and uses this information to allocate MMIO +space for the BARs. That MMIO space is then setup to be +associated with the host bridge so that it works when generic +PCI subsystem code in Linux processes the BARs. + +Finally, hv_pci_probe() creates the root PCI bus. At this +point the Hyper-V virtual PCI driver hackery is done, and the +normal Linux PCI machinery for scanning the root bus works to +detect the device, to perform driver matching, and to +initialize the driver and device. + +PCI Device Removal +------------------ +A Hyper-V host may initiate removal of a vPCI device from a +guest VM at any time during the life of the VM. The removal +is instigated by an admin action taken on the Hyper-V host and +is not under the control of the guest OS. + +A guest VM is notified of the removal by an unsolicited +"Eject" message sent from the host to the guest over the VMBus +channel associated with the vPCI device. Upon receipt of such +a message, the Hyper-V virtual PCI driver in Linux +asynchronously invokes Linux kernel PCI subsystem calls to +shutdown and remove the device. When those calls are +complete, an "Ejection Complete" message is sent back to +Hyper-V over the VMBus channel indicating that the device has +been removed. At this point, Hyper-V sends a VMBus rescind +message to the Linux guest, which the VMBus driver in Linux +processes by removing the VMBus identity for the device. Once +that processing is complete, all vestiges of the device having +been present are gone from the Linux kernel. The rescind +message also indicates to the guest that Hyper-V has stopped +providing support for the vPCI device in the guest. If the +guest were to attempt to access that device's MMIO space, it +would be an invalid reference. Hypercalls affecting the device +return errors, and any further messages sent in the VMBus +channel are ignored. + +After sending the Eject message, Hyper-V allows the guest VM +60 seconds to cleanly shutdown the device and respond with +Ejection Complete before sending the VMBus rescind +message. If for any reason the Eject steps don't complete +within the allowed 60 seconds, the Hyper-V host forcibly +performs the rescind steps, which will likely result in +cascading errors in the guest because the device is now no +longer present from the guest standpoint and accessing the +device MMIO space will fail. + +Because ejection is asynchronous and can happen at any point +during the guest VM lifecycle, proper synchronization in the +Hyper-V virtual PCI driver is very tricky. Ejection has been +observed even before a newly offered vPCI device has been +fully setup. The Hyper-V virtual PCI driver has been updated +several times over the years to fix race conditions when +ejections happen at inopportune times. Care must be taken when +modifying this code to prevent re-introducing such problems. +See comments in the code. + +Interrupt Assignment +-------------------- +The Hyper-V virtual PCI driver supports vPCI devices using +MSI, multi-MSI, or MSI-X. Assigning the guest vCPU that will +receive the interrupt for a particular MSI or MSI-X message is +complex because of the way the Linux setup of IRQs maps onto +the Hyper-V interfaces. For the single-MSI and MSI-X cases, +Linux calls hv_compse_msi_msg() twice, with the first call +containing a dummy vCPU and the second call containing the +real vCPU. Furthermore, hv_irq_unmask() is finally called +(on x86) or the GICD registers are set (on arm64) to specify +the real vCPU again. Each of these three calls interact +with Hyper-V, which must decide which physical CPU should +receive the interrupt before it is forwarded to the guest VM. +Unfortunately, the Hyper-V decision-making process is a bit +limited, and can result in concentrating the physical +interrupts on a single CPU, causing a performance bottleneck. +See details about how this is resolved in the extensive +comment above the function hv_compose_msi_req_get_cpu(). + +The Hyper-V virtual PCI driver implements the +irq_chip.irq_compose_msi_msg function as hv_compose_msi_msg(). +Unfortunately, on Hyper-V the implementation requires sending +a VMBus message to the Hyper-V host and awaiting an interrupt +indicating receipt of a reply message. Since +irq_chip.irq_compose_msi_msg can be called with IRQ locks +held, it doesn't work to do the normal sleep until awakened by +the interrupt. Instead hv_compose_msi_msg() must send the +VMBus message, and then poll for the completion message. As +further complexity, the vPCI device could be ejected/rescinded +while the polling is in progress, so this scenario must be +detected as well. See comments in the code regarding this +very tricky area. + +Most of the code in the Hyper-V virtual PCI driver (pci- +hyperv.c) applies to Hyper-V and Linux guests running on x86 +and on arm64 architectures. But there are differences in how +interrupt assignments are managed. On x86, the Hyper-V +virtual PCI driver in the guest must make a hypercall to tell +Hyper-V which guest vCPU should be interrupted by each +MSI/MSI-X interrupt, and the x86 interrupt vector number that +the x86_vector IRQ domain has picked for the interrupt. This +hypercall is made by hv_arch_irq_unmask(). On arm64, the +Hyper-V virtual PCI driver manages the allocation of an SPI +for each MSI/MSI-X interrupt. The Hyper-V virtual PCI driver +stores the allocated SPI in the architectural GICD registers, +which Hyper-V emulates, so no hypercall is necessary as with +x86. Hyper-V does not support using LPIs for vPCI devices in +arm64 guest VMs because it does not emulate a GICv3 ITS. + +The Hyper-V virtual PCI driver in Linux supports vPCI devices +whose drivers create managed or unmanaged Linux IRQs. If the +smp_affinity for an unmanaged IRQ is updated via the /proc/irq +interface, the Hyper-V virtual PCI driver is called to tell +the Hyper-V host to change the interrupt targeting and +everything works properly. However, on x86 if the x86_vector +IRQ domain needs to reassign an interrupt vector due to +running out of vectors on a CPU, there's no path to inform the +Hyper-V host of the change, and things break. Fortunately, +guest VMs operate in a constrained device environment where +using all the vectors on a CPU doesn't happen. Since such a +problem is only a theoretical concern rather than a practical +concern, it has been left unaddressed. + +DMA +--- +By default, Hyper-V pins all guest VM memory in the host +when the VM is created, and programs the physical IOMMU to +allow the VM to have DMA access to all its memory. Hence +it is safe to assign PCI devices to the VM, and allow the +guest operating system to program the DMA transfers. The +physical IOMMU prevents a malicious guest from initiating +DMA to memory belonging to the host or to other VMs on the +host. From the Linux guest standpoint, such DMA transfers +are in "direct" mode since Hyper-V does not provide a virtual +IOMMU in the guest. + +Hyper-V assumes that physical PCI devices always perform +cache-coherent DMA. When running on x86, this behavior is +required by the architecture. When running on arm64, the +architecture allows for both cache-coherent and +non-cache-coherent devices, with the behavior of each device +specified in the ACPI DSDT. But when a PCI device is assigned +to a guest VM, that device does not appear in the DSDT, so the +Hyper-V VMBus driver propagates cache-coherency information +from the VMBus node in the ACPI DSDT to all VMBus devices, +including vPCI devices (since they have a dual identity as a VMBus +device and as a PCI device). See vmbus_dma_configure(). +Current Hyper-V versions always indicate that the VMBus is +cache coherent, so vPCI devices on arm64 always get marked as +cache coherent and the CPU does not perform any sync +operations as part of dma_map/unmap_*() calls. + +vPCI protocol versions +---------------------- +As previously described, during vPCI device setup and teardown +messages are passed over a VMBus channel between the Hyper-V +host and the Hyper-v vPCI driver in the Linux guest. Some +messages have been revised in newer versions of Hyper-V, so +the guest and host must agree on the vPCI protocol version to +be used. The version is negotiated when communication over +the VMBus channel is first established. See +hv_pci_protocol_negotiation(). Newer versions of the protocol +extend support to VMs with more than 64 vCPUs, and provide +additional information about the vPCI device, such as the +guest virtual NUMA node to which it is most closely affined in +the underlying hardware. + +Guest NUMA node affinity +------------------------ +When the vPCI protocol version provides it, the guest NUMA +node affinity of the vPCI device is stored as part of the Linux +device information for subsequent use by the Linux driver. See +hv_pci_assign_numa_node(). If the negotiated protocol version +does not support the host providing NUMA affinity information, +the Linux guest defaults the device NUMA node to 0. But even +when the negotiated protocol version includes NUMA affinity +information, the ability of the host to provide such +information depends on certain host configuration options. If +the guest receives NUMA node value "0", it could mean NUMA +node 0, or it could mean "no information is available". +Unfortunately it is not possible to distinguish the two cases +from the guest side. + +PCI config space access in a CoCo VM +------------------------------------ +Linux PCI device drivers access PCI config space using a +standard set of functions provided by the Linux PCI subsystem. +In Hyper-V guests these standard functions map to functions +hv_pcifront_read_config() and hv_pcifront_write_config() +in the Hyper-V virtual PCI driver. In normal VMs, +these hv_pcifront_*() functions directly access the PCI config +space, and the accesses trap to Hyper-V to be handled. +But in CoCo VMs, memory encryption prevents Hyper-V +from reading the guest instruction stream to emulate the +access, so the hv_pcifront_*() functions must invoke +hypercalls with explicit arguments describing the access to be +made. + +Config Block back-channel +------------------------- +The Hyper-V host and Hyper-V virtual PCI driver in Linux +together implement a non-standard back-channel communication +path between the host and guest. The back-channel path uses +messages sent over the VMBus channel associated with the vPCI +device. The functions hyperv_read_cfg_blk() and +hyperv_write_cfg_blk() are the primary interfaces provided to +other parts of the Linux kernel. As of this writing, these +interfaces are used only by the Mellanox mlx5 driver to pass +diagnostic data to a Hyper-V host running in the Azure public +cloud. The functions hyperv_read_cfg_blk() and +hyperv_write_cfg_blk() are implemented in a separate module +(pci-hyperv-intf.c, under CONFIG_PCI_HYPERV_INTERFACE) that +effectively stubs them out when running in non-Hyper-V +environments. |