On Wed, Jan 06, 2021 at 08:49:32PM +0000, Dexuan Cui wrote: > > From: Michael Kelley <mikelley@xxxxxxxxxxxxx> > > Sent: Wednesday, January 6, 2021 9:38 AM > > From: Dexuan Cui <decui@xxxxxxxxxxxxx> > > Sent: Tuesday, December 22, 2020 4:12 PM > > > > > > When a Linux VM runs on Hyper-V, if the host toolstack doesn't support > > > hibernation for the VM (this happens on old Hyper-V hosts like Windows > > > Server 2016, or new Hyper-V hosts if the admin or user doesn't declare > > > the hibernation intent for the VM), the VM is discouraged from trying > > > hibernation (because the host doesn't guarantee that the VM's virtual > > > hardware configuration will remain exactly the same across hibernation), > > > i.e. the VM should not try to set up the swap partition/file for > > > hibernation, etc. > > > > > > x86 Hyper-V uses the presence of the virtual ACPI S4 state as the > > > indication of the host toolstack support for a VM. Currently there is > > > no easy and reliable way for the userspace to detect the presence of > > > the state (see ...). Add > > > /sys/bus/vmbus/supported_features for this purpose. > > > > I'm OK with surfacing the hibernation capability via an entry in > > /sys/bus/vmbus. Correct me if I'm wrong, but I think the concept > > being surfaced is not "ACPI S4 state" precisely, but slightly more > > generally whether hibernation is supported for the VM. While > > those two concepts may be 1:1 for the moment, there might be > > future configurations where "hibernation is supported" depends > > on other factors as well. > > For x86, I believe the virtual ACPI S4 state exists only when the > admin/user declares the intent of "enable hibernation for the VM" via > some PowwerShell/WMI command. On Azure, if a VM size is not suitable > for hibernation (e.g. an existing VM has an ephemeral local disk), > the toolstack on the host should not enable the ACPI S4 state for the > VM. That's why we implemented hv_is_hibernation_supported() for x86 by > checking the ACPI S4 state, and we have used the function > hv_is_hibernation_supported() in hv_utils and hv_balloon for quite a > while. > > For ARM, IIRC there is no concept of ACPI S4 state, so currently > hv_is_hibernation_supported() is actually not implemented. Not sure Because the core support for ARM64 Hyper-V is not merged yet. In Michael's core patchset, hv_is_hibernation_supported() is implemented as always returning false, and there is more work (other than Michael's core pachset) to make hiberation work on ARM64 Hyper-V guest. Regards, Boqun > why hv_utils and hv_balloon can build successfully... :-) Probably > Boqun can help to take a look. > > > > > The guidance for things in /sys is that they generally should > > be single valued (see Documentation/filesystems/sysfs.rst). So my > > recommendation is to create a "hibernation" entry that has a value > > of 0 or 1. > > > > Michael > > Got it. Then let's use /sys/bus/vmbus/hibernation. > > Will post v3. > > Thanks, > -- Dexuan >
