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Re: [Qemu-devel] [PATCH v8 4/8] ACPI: Add Virtual Machine Generation ID

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v8 4/8] ACPI: Add Virtual Machine Generation ID support
Date: Mon, 20 Feb 2017 14:00:39 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 02/20/2017 04:23 AM, Dr. David Alan Gilbert wrote:
> * Laszlo Ersek (address@hidden) wrote:
>> CC Dave
> This isn't an area I really understand; but if I'm
> reading this right then 
>    vmgenid is stored in fw_cfg?
>    fw_cfg isn't migrated
> So why should any changes to it get migrated, except if it's already
> been read by the guest (and if the guest reads it again aftwards what's
> it expected to read?)

Why are we expecting it to change on migration?  You want a new value
when you load state from disk (you don't know how many times the same
state has been loaded previously, so each load is effectively forking
the VM and you want a different value), but for a single live migration,
you aren't forking the VM and don't need a new generation ID.

I guess it all boils down to what command line you're using: if libvirt
is driving a live migration, it will request the same UUID in the
command line of the destination as what is on the source; while if
libvirt is loading from a [managed]save to restore state from a file, it
will either request a new UUID directly or request auto to let qemu
generate the new id.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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