Re: [PATCH v2 1/1] Documentation: hyperv: Add overview of guest VM hibernation

[Bagas Sanjaya <bagasdotme@gmail.com>]

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On Wed, Jan 08, 2025 at 04:50:15AM +0000, Michael Kelley wrote:
> From: Bagas Sanjaya <bagasdotme@gmail.com> Sent: Tuesday, January 7, 2025 8:07 PM
> > 
> > On Tue, Jan 07, 2025 at 12:20:47PM -0800, mhkelley58@gmail.com wrote:
> > > +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.
> > 
> > Did you mean for example, conflicting synthetic NICs?
> 
> In the resumed hibernation image, the unmatched device is in a weird
> state where it is exist and has a driver, but is no longer "open" in the VMBus
> layer. Any attempt to do I/O to the device will fail, and interrupts received
> from the device are ignored. Presumably there's user space software or a
> network connection that has the device open and expects to be able to
> interact with it. That software will error out due to the I/O failure.
> 
> I haven't thought through all the implications of such a scenario, so
> just left the documentation as "likely to cause errors" without going
> into detail. It's an unsupported scenario, so not likely something that
> will be improved.
> 
> I don't think the issue is necessarily conflicting NICs, though if a NIC with
> a different instance GUID was offered, it would show up as a new NIC
> in the resumed image, and that might cause conflicts/confusion with
> the "dead" NIC.

Understand.

> 
> > 
> > > +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.
> > 
> > Nothing can be done on host-side?
> 
> Not really.  Whatever host-side software that is using the Hyper-V
> socket will just get an error that next time it tries to do I/O over
> the socket.
> 
> Is there something you had in mind that the host could/should do?

None really.

> 
> > 
> > > +Virtual PCI devices are physical PCI devices that are mapped directly
> > > +into the VM's physical address space so the VM can interact directly
> > > +the hardware. vPCI devices include those accessed via what Hyper-V
> > "... interact directly with the hardware."
> 
> Thanks for your careful reading.  I'll add the missing "with".  :-)
> 
> > > +calls "Discrete Device Assignment" (DDA), as well as SR-IOV NIC
> > > +Virtual Functions (VF) devices. See Documentation/virt/hyperv/vpci.rst.
> > > +
> > > <snipped>...
> > > +SR-IOV NIC VFs similarly 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, if the VMBus connection is
> > > +unloaded and then re-established without the VM being rebooted (as
> > > +happens in Steps 3 and 5 in the Detailed Hibernation Sequence above,
> > > +and similarly in the Detailed Resume Sequence), VFs are already part
> >                                                   "... that are already ..."
> 
> Right. I'll fix this wording problem as well.

OK, thanks!

-- 
An old man doll... just what I always wanted! - Clara

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