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Re: Live migration and PV device handling

  • To: Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 6 Apr 2020 18:24:06 +0100
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  • Cc: Xen-devel <xen-devel@xxxxxxxxxxxxx>, Anastassios Nanos <anastassios.nanos@xxxxxxxxxxx>
  • Delivery-date: Mon, 06 Apr 2020 17:24:16 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

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.




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