On Sun, Jan 12, 2025 at 09:29:03PM -0800, mhkelley58@xxxxxxxxx wrote: > From: Michael Kelley <mhklinux@xxxxxxxxxxx> > > Add documentation on how hibernation works in a guest VM on Hyper-V. > Describe how VMBus devices and the VMBus itself are hibernated and > resumed, along with various limitations. > > Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx> > --- > Changes in v3: > * Added missing word "with" in vPCI section [Bagas Sanjaya] > * Reworked wording of SR-IOV NIC handling [Bagas Sanjaya] > > Changes in v2: > * Added discussion of implications of moving a hibernated VM to another > Hyper-V host and resuming on the new host [Roman Kisel] > * Added section describing how UIO devices prevent a VM from being > hibernated [Roman Kisel] > > Documentation/virt/hyperv/hibernation.rst | 336 ++++++++++++++++++++++ You forget to add the doc to toctree index: Documentation/virt/hyperv/hibernation.rst: WARNING: document isn't included in any toctree > 1 file changed, 336 insertions(+) > create mode 100644 Documentation/virt/hyperv/hibernation.rst > > diff --git a/Documentation/virt/hyperv/hibernation.rst b/Documentation/virt/hyperv/hibernation.rst > new file mode 100644 > index 000000000000..4ff27f4a317a > --- /dev/null > +++ b/Documentation/virt/hyperv/hibernation.rst > @@ -0,0 +1,336 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +Hibernating Guest VMs > +===================== > + > +Background > +---------- > +Linux supports the ability to hibernate itself in order to save power. > +Hibernation is sometimes called suspend-to-disk, as it writes a memory > +image to disk and puts the hardware into the lowest possible power > +state. Upon resume from hibernation, the hardware is restarted and the > +memory image is restored from disk so that it can resume execution > +where it left off. See the "Hibernation" section of > +Documentation/admin-guide/pm/sleep-states.rst. > + > +Hibernation is usually done on devices with a single user, such as a > +personal laptop. For example, the laptop goes into hibernation when > +the cover is closed, and resumes when the cover is opened again. > +Hibernation and resume happen on the same hardware, and Linux kernel > +code orchestrating the hibernation steps assumes that the hardware > +configuration is not changed while in the hibernated state. > + > +Hibernation can be initiated within Linux by writing "disk" to > +/sys/power/state or by invoking the reboot system call with the > +appropriate arguments. This functionality may be wrapped by user space > +commands such "systemctl hibernate" that are run directly from a > +command line or in response to events such as the laptop lid closing. > + > +Considerations for Guest VM Hibernation > +--------------------------------------- > +Linux guests on Hyper-V can also be hibernated, in which case the > +hardware is the virtual hardware provided by Hyper-V to the guest VM. > +Only the targeted guest VM is hibernated, while other guest VMs and > +the underlying Hyper-V host continue to run normally. While the > +underlying Windows Hyper-V and physical hardware on which it is > +running might also be hibernated using hibernation functionality in > +the Windows host, host hibernation and its impact on guest VMs is not > +in scope for this documentation. > + > +Resuming a hibernated guest VM can be more challenging than with > +physical hardware because VMs make it very easy to change the hardware > +configuration between the hibernation and resume. Even when the resume > +is done on the same VM that hibernated, the memory size might be > +changed, or virtual NICs or SCSI controllers might be added or > +removed. Virtual PCI devices assigned to the VM might be added or > +removed. Most such changes cause the resume steps to fail, though > +adding a new virtual NIC, SCSI controller, or vPCI device should work. > + > +Additional complexity can ensue because the disks of the hibernated VM > +can be moved to another newly created VM that otherwise has the same > +virtual hardware configuration. While it is desirable for resume from > +hibernation to succeed after such a move, there are challenges. See > +details on this scenario and its limitations in the "Resuming on a > +Different VM" section below. > + > +Hyper-V also provides ways to move a VM from one Hyper-V host to > +another. Hyper-V tries to ensure processor model and Hyper-V version > +compatibility using VM Configuration Versions, and prevents moves to > +a host that isn't compatible. Linux adapts to host and processor > +differences by detecting them at boot time, but such detection is not > +done when resuming execution in the hibernation image. If a VM is > +hibernated on one host, then resumed on a host with a different processor > +model or Hyper-V version, settings recorded in the hibernation image > +may not match the new host. Because Linux does not detect such > +mismatches when resuming the hibernation image, undefined behavior > +and failures could result. > + > + > +Enabling Guest VM Hibernation > +----------------------------- > +Hibernation of a Hyper-V guest VM is disabled by default because > +hibernation is incompatible with memory hot-add, as provided by the > +Hyper-V balloon driver. If hot-add is used and the VM hibernates, it > +hibernates with more memory than it started with. But when the VM > +resumes from hibernation, Hyper-V gives the VM only the originally > +assigned memory, and the memory size mismatch causes resume to fail. > + > +To enable a Hyper-V VM for hibernation, the Hyper-V administrator must > +enable the ACPI virtual S4 sleep state in the ACPI configuration that > +Hyper-V provides to the guest VM. Such enablement is accomplished by > +modifying a WMI property of the VM, the steps for which are outside > +the scope of this documentation but are available on the web. > +Enablement is treated as the indicator that the administrator > +prioritizes Linux hibernation in the VM over hot-add, so the Hyper-V > +balloon driver in Linux disables hot-add. Enablement is indicated if > +the contents of /sys/power/disk contains "platform" as an option. The > +enablement is also visible in /sys/bus/vmbus/hibernation. See function > +hv_is_hibernation_supported(). > + > +Linux supports ACPI sleep states on x86, but not on arm64. So Linux > +guest VM hibernation is not available on Hyper-V for arm64. > + > +Initiating Guest VM Hibernation > +------------------------------- > +Guest VMs can self-initiate hibernation using the standard Linux > +methods of writing "disk" to /sys/power/state or the reboot system > +call. As an additional layer, Linux guests on Hyper-V support the > +"Shutdown" integration service, via which a Hyper-V administrator can > +tell a Linux VM to hibernate using a command outside the VM. The > +command generates a request to the Hyper-V shutdown driver in Linux, > +which sends the uevent "EVENT=hibernate". See kernel functions > +shutdown_onchannelcallback() and send_hibernate_uevent(). A udev rule > +must be provided in the VM that handles this event and initiates > +hibernation. > + > +Handling VMBus Devices During Hibernation & Resume > +-------------------------------------------------- > +The VMBus bus driver, and the individual VMBus device drivers, > +implement suspend and resume functions that are called as part of the > +Linux orchestration of hibernation and of resuming from hibernation. > +The overall approach is to leave in place the data structures for the > +primary VMBus channels and their associated Linux devices, such as > +SCSI controllers and others, so that they are captured in the > +hibernation image. This approach allows any state associated with the > +device to be persisted across the hibernation/resume. When the VM > +resumes, the devices are re-offered by Hyper-V and are connected to > +the data structures that already exist in the resumed hibernation > +image. > + > +VMBus devices are identified by class and instance GUID. (See section > +"VMBus device creation/deletion" in > +Documentation/virt/hyperv/vmbus.rst.) Upon resume from hibernation, > +the resume functions expect that the devices offered by Hyper-V have > +the same class/instance GUIDs as the devices present at the time of > +hibernation. Having the same class/instance GUIDs allows the offered > +devices to be matched to the primary VMBus channel data structures in > +the memory of the now resumed hibernation image. If any devices are > +offered that don't match primary VMBus channel data structures that > +already exist, they are processed normally as newly added devices. If > +primary VMBus channels that exist in the resumed hibernation image are > +not matched with a device offered in the resumed VM, the resume > +sequence waits for 10 seconds, then proceeds. But the unmatched device > +is likely to cause errors in the resumed VM. > + > +When resuming existing primary VMBus channels, the newly offered > +relids might be different because relids can change on each VM boot, > +even if the VM configuration hasn't changed. The VMBus bus driver > +resume function matches the class/instance GUIDs, and updates the > +relids in case they have changed. > + > +VMBus sub-channels are not persisted in the hibernation image. Each > +VMBus device driver's suspend function must close any sub-channels > +prior to hibernation. Closing a sub-channel causes Hyper-V to send a > +RESCIND_CHANNELOFFER message, which Linux processes by freeing the > +channel data structures so that all vestiges of the sub-channel are > +removed. By contrast, primary channels are marked closed and their > +ring buffers are freed, but Hyper-V does not send a rescind message, > +so the channel data structure continues to exist. Upon resume, the > +device driver's resume function re-allocates the ring buffer and > +re-opens the existing channel. It then communicates with Hyper-V to > +re-open sub-channels from scratch. > + > +The Linux ends of Hyper-V sockets are forced closed at the time of > +hibernation. The guest can't force closing the host end of the socket, > +but any host-side actions on the host end will produce an error. > + > +VMBus devices use the same suspend function for the "freeze" and the > +"poweroff" phases, and the same resume function for the "thaw" and > +"restore" phases. See the "Entering Hibernation" section of > +Documentation/driver-api/pm/devices.rst for the sequencing of the > +phases. > + > +Detailed Hibernation Sequence > +----------------------------- > +1. The Linux power management (PM) subsystem prepares for > + hibernation by freezing user space processes and allocating > + memory to hold the hibernation image. > +2. As part of the "freeze" phase, Linux PM calls the "suspend" > + function for each VMBus device in turn. As described above, this > + function removes sub-channels, and leaves the primary channel in > + a closed state. > +3. Linux PM calls the "suspend" function for the VMBus bus, which > + closes any Hyper-V socket channels and unloads the top-level > + VMBus connection with the Hyper-V host. > +4. Linux PM disables non-boot CPUs, creates the hibernation image in > + the previously allocated memory, then re-enables non-boot CPUs. > + The hibernation image contains the memory data structures for the > + closed primary channels, but no sub-channels. > +5. As part of the "thaw" phase, Linux PM calls the "resume" function > + for the VMBus bus, which re-establishes the top-level VMBus > + connection and requests that Hyper-V re-offer the VMBus devices. > + As offers are received for the primary channels, the relids are > + updated as previously described. > +6. Linux PM calls the "resume" function for each VMBus device. Each > + device re-opens its primary channel, and communicates with Hyper-V > + to re-establish sub-channels if appropriate. The sub-channels > + are re-created as new channels since they were previously removed > + entirely in Step 2. > +7. With VMBus devices now working again, Linux PM writes the > + hibernation image from memory to disk. > +8. Linux PM repeats Steps 2 and 3 above as part of the "poweroff" > + phase. VMBus channels are closed and the top-level VMBus > + connection is unloaded. > +9. Linux PM disables non-boot CPUs, and then enters ACPI sleep state > + S4. Hibernation is now complete. > + > +Detailed Resume Sequence > +------------------------ > +1. The guest VM boots into a fresh Linux OS instance. During boot, > + the top-level VMBus connection is established, and synthetic > + devices are enabled. This happens via the normal paths that don't > + involve hibernation. > +2. Linux PM hibernation code reads swap space is to find and read > + the hibernation image into memory. If there is no hibernation > + image, then this boot becomes a normal boot. > +3. If this is a resume from hibernation, the "freeze" phase is used > + to shutdown VMBus devices and unload the top-level VMBus > + connection in the running fresh OS instance, just like Steps 2 > + and 3 in the hibernation sequence. > +4. Linux PM disables non-boot CPUs, and transfers control to the > + read-in hibernation image. In the now-running hibernation image, > + non-boot CPUs are restarted. > +5. As part of the "resume" phase, Linux PM repeats Steps 5 and 6 > + from the hibernation sequence. The top-level VMBus connection is > + re-established, and offers are received and matched to primary > + channels in the image. Relids are updated. VMBus device resume > + functions re-open primary channels and re-create sub-channels. > +6. Linux PM exits the hibernation resume sequence and the VM is now > + running normally from the hibernation image. > + > +Key-Value Pair (KVP) Pseudo-Device Anomalies > +-------------------------------------------- > +The VMBus KVP device behaves differently from other pseudo-devices > +offered by Hyper-V. When the KVP primary channel is closed, Hyper-V > +sends a rescind message, which causes all vestiges of the device to be > +removed. But Hyper-V then re-offers the device, causing it to be newly > +re-created. The removal and re-creation occurs during the "freeze" > +phase of hibernation, so the hibernation image contains the re-created > +KVP device. Similar behavior occurs during the "freeze" phase of the > +resume sequence while still in the fresh OS instance. But in both > +cases, the top-level VMBus connection is subsequently unloaded, which > +causes the device to be discarded on the Hyper-V side. So no harm is > +done and everything still works. > + > +Virtual PCI devices > +------------------- > +Virtual PCI devices are physical PCI devices that are mapped directly > +into the VM's physical address space so the VM can interact directly > +with the hardware. vPCI devices include those accessed via what Hyper-V > +calls "Discrete Device Assignment" (DDA), as well as SR-IOV NIC > +Virtual Functions (VF) devices. See Documentation/virt/hyperv/vpci.rst. > + > +Hyper-V DDA devices are offered to guest VMs after the top-level VMBus > +connection is established, just like VMBus synthetic devices. They are > +statically assigned to the VM, and their instance GUIDs don't change > +unless the Hyper-V administrator makes changes to the configuration. > +DDA devices are represented in Linux as virtual PCI devices that have > +a VMBus identity as well as a PCI identity. Consequently, Linux guest > +hibernation first handles DDA devices as VMBus devices in order to > +manage the VMBus channel. But then they are also handled as PCI > +devices using the hibernation functions implemented by their native > +PCI driver. > + > +SR-IOV NIC VFs also have a VMBus identity as well as a PCI > +identity, and overall are processed similarly to DDA devices. A > +difference is that VFs are not offered to the VM during initial boot > +of the VM. Instead, the VMBus synthetic NIC driver first starts > +operating and communicates to Hyper-V that it is prepared to accept a > +VF, and then the VF offer is made. However, the VMBus connection > +might later be unloaded and then re-established without the VM being > +rebooted, as happens in Steps 3 and 5 in the Detailed Hibernation > +Sequence above and in the Detailed Resume Sequence. In such a case, > +the VFs likely became part of the VM during initial boot, so when the > +VMBus connection is re-established, the VFs are offered on the > +re-established connection without intervention by the synthetic NIC driver. > + > +UIO Devices > +----------- > +A VMBus device can be exposed to user space using the Hyper-V UIO > +driver (uio_hv_generic.c) so that a user space driver can control and > +operate the device. However, the VMBus UIO driver does not support the > +suspend and resume operations needed for hibernation. If a VMBus > +device is configured to use the UIO driver, hibernating the VM fails > +and Linux continues to run normally. The most common use of the Hyper-V > +UIO driver is for DPDK networking, but there are other uses as well. > + > +Resuming on a Different VM > +-------------------------- > +This scenario occurs in the Azure public cloud in that a hibernated > +customer VM only exists as saved configuration and disks -- the VM no > +longer exists on any Hyper-V host. When the customer VM is resumed, a > +new Hyper-V VM with identical configuration is created, likely on a > +different Hyper-V host. That new Hyper-V VM becomes the resumed > +customer VM, and the steps the Linux kernel takes to resume from the > +hibernation image must work in that new VM. > + > +While the disks and their contents are preserved from the original VM, > +the Hyper-V-provided VMBus instance GUIDs of the disk controllers and > +other synthetic devices would typically be different. The difference > +would cause the resume from hibernation to fail, so several things are > +done to solve this problem: > + > +* For VMBus synthetic devices that support only a single instance, > + Hyper-V always assigns the same instance GUIDs. For example, the > + Hyper-V mouse, the shutdown pseudo-device, the time sync pseudo > + device, etc., always have the same instance GUID, both for local > + Hyper-V installs as well as in the Azure cloud. > + > +* VMBus synthetic SCSI controllers may have multiple instances in a > + VM, and in the general case instance GUIDs vary from VM to VM. > + However, Azure VMs always have exactly two synthetic SCSI > + controllers, and Azure code overrides the normal Hyper-V behavior > + so these controllers are always assigned the same two instance > + GUIDs. Consequently, when a customer VM is resumed on a newly > + created VM, the instance GUIDs match. But this guarantee does not > + hold for local Hyper-V installs. > + > +* Similarly, VMBus synthetic NICs may have multiple instances in a > + VM, and the instance GUIDs vary from VM to VM. Again, Azure code > + overrides the normal Hyper-V behavior so that the instance GUID > + of a synthetic NIC in a customer VM does not change, even if the > + customer VM is deallocated or hibernated, and then re-constituted > + on a newly created VM. As with SCSI controllers, this behavior > + does not hold for local Hyper-V installs. > + > +* vPCI devices do not have the same instance GUIDs when resuming > + from hibernation on a newly created VM. Consequently, Azure does > + not support hibernation for VMs that have DDA devices such as > + NVMe controllers or GPUs. For SR-IOV NIC VFs, Azure removes the > + VF from the VM before it hibernates so that the hibernation image > + does not contain a VF device. When the VM is resumed it > + instantiates a new VF, rather than trying to match against a VF > + that is present in the hibernation image. Because Azure must > + remove any VFs before initiating hibernation, Azure VM > + hibernation must be initiated externally from the Azure Portal or > + Azure CLI, which in turn uses the Shutdown integration service to > + tell Linux to do the hibernation. If hibernation is self-initiated > + within the Azure VM, VFs remain in the hibernation image, and are > + not resumed properly. > + > +In summary, Azure takes special actions to remove VFs and to ensure > +that VMBus device instance GUIDs match on a new/different VM, allowing > +hibernation to work for most general-purpose Azure VMs sizes. While > +similar special actions could be taken when resuming on a different VM > +on a local Hyper-V install, orchestrating such actions is not provided > +out-of-the-box by local Hyper-V and so requires custom scripting. The docs itself LGTM. Thanks. -- An old man doll... just what I always wanted! - Clara
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