Re: [PATCH] Drivers: hv: vmbus: Add /sys/bus/vmbus/supported_features

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Jan 06, 2021 at 08:49:32PM +0000, Dexuan Cui wrote:
> > From: Michael Kelley <mikelley@microsoft.com>
> > Sent: Wednesday, January 6, 2021 9:38 AM
> > From: Dexuan Cui <decui@microsoft.com>
> > 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
>