Re: [PATCH] target/i386: always create kvmclock device

[Vitaly Kuznetsov]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:

> * Antoine Damhet (antoine.damhet@blade-group.com) wrote:
>> On Thu, Sep 17, 2020 at 06:44:10PM +0100, Dr. David Alan Gilbert wrote:
>> 
>> [...]
>> 
>> > > >> >
>> > > >> > Shouldn't the old check used when machine type <= 5.1 in order to 
>> > > >> > avoid
>> > > >> > migration incompatibility ?
>> > > >> 
>> > > >> Hm, when the check fails we just don't create the device and no error 
>> > > >> is
>> > > >> reported, so even if we have kvmclock data in the migration stream but
>> > > >> fail to create it migration will still succeed, right? (not a 
>> > > >> migration
>> > > >> expert here :-)
>> > > >
>> > > > When the migration stream is parsed, it'll try and find a "kvmclock"
>> > > > device to pass the data it's reading to; if one doesn't exist it'll
>> > > > fail.
>> > > 
>> > > This may happen with an older machine type when the destination is
>> > > running an unfixed QEMU and the source has the fix, right?
>> > 
>> > Yes I think so.
>> > 
>> > > The solution
>> > > would be to introduce a flag for older machine types (or for new ones)
>> > > like 'kvmclock_always'.
>> > 
>> > Yep sounds the normal answer.
>> > (You might want to try it first to trigger the bug)
>> 
>> So, I tried the patch and:
>> 
>> # patched -> patched
>> 
>> Everything working as expected
>> 
>> # patched -> unpatched
>> 
>> Migration failure with:
>> 
>> ```
>> Unknown savevm section or instance 'kvmclock' 0. Make sure that your current 
>> VM setup matches your saved VM setup, including any hotplugged devices
>> load of migration failed: Invalid argument
>> ```
>
> Right, that's what I expected and said we need to wire this fix to the
> machine type.
>

v2 with the idea implemented is coming. As I'm not a regular contributor
to machine types, please review thoroughly :-)

-- 
Vitaly