Re: Live migration and PV device handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/04/2020 18:16, Tamas K Lengyel wrote:
> On Fri, Apr 3, 2020 at 6:44 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> On 03/04/2020 13:32, Anastassios Nanos wrote:
>>> Hi all,
>>>
>>> I am trying to understand how live-migration happens in xen. I am
>>> looking in the HVM guest case and I have dug into the relevant parts
>>> of the toolstack and the hypervisor regarding memory, vCPU context
>>> etc.
>>>
>>> In particular, I am interested in how PV device migration happens. I
>>> assume that the guest is not aware of any suspend/resume operations
>>> being done
>> Sadly, this assumption is not correct.  HVM guests with PV drivers
>> currently have to be aware in exactly the same way as PV guests.
>>
>> Work is in progress to try and address this.  See
>> https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=775a02452ddf3a6889690de90b1a94eb29c3c732
>> (sorry - for some reason that doc isn't being rendered properly in
>> https://xenbits.xen.org/docs/ )
> That proposal is very interesting - first time it came across my radar
> - but I dislike the idea that domain IDs need to be preserved for
> uncooperative migration to work.

The above restriction is necessary to work with existing guests, which
is an implementation requirement of the folks driving the work.

> Ideally I would be able to take
> advantage of the same plumbing to perform forking of VMs with PV
> drivers where preserving the domain id is impossible since its still
> in use.

We would of course like to make changes to remove the above restriction
in the longterm.  The problem is that it is not a trivial thing to fix. 
Various things were discussed in Chicago, but I don't recall if any of
the plans made their way onto xen-devel.

~Andrew