Re: [Xen-devel] live migrating hvm from 4.4 to 4.5 fails due to kvmvapic

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 2 Aug 2016, Olaf Hering wrote:
> As a followup to the issue below, and the one which "just" popped in in
> qemu-2.6+:
> 
> Why is the machine description for xen not fixed?

xenfv comes from a time when QEMU didn't have machine description
versioning. To have versioning, it is possible to use a regular PC
machine plus accel=xen. I tried to change the libxl default from xenfv
to a versioned pc machine with accel=xen a couple of years back, but
unfortunately it created ABI incompatibilities, see:

http://marc.info/?i=alpine.DEB.2.02.1406121512360.13771%40kaball.uk.xensource.com


> Shouldnt the be some sort of verification of old and new 'xenfv' when a
> new qemu rc1 is done?

It would be great to have


> Is there a way to dump the machine description in text form?

I don't know, but people at qemu-devel might.

 
> On Fri, May 13, Stefano Stabellini wrote:
> 
> > On Thu, 12 May 2016, Olaf Hering wrote:
> > > On Thu, May 12, Olaf Hering wrote:
> > > 
> > > > One thing to fix it in staging-4.5 is to introduce a dummy device which
> > > > handles a section named "kvm-tpr-opt". I already have a hack which does
> > > > that, and the migration proceeds. I will propose a patch to deal with
> > > > this part of the bug.
> > > 
> > > Something like shown below.
> > 
> > Thanks for looking into this. I don't think that adding a dummy device
> > in QEMU is acceptable. This kind of problems is usually solved with
> > versioning the PC machine in QEMU, see all the pc_machine_* in
> > hw/i386/pc_piix.c. One specific version of the machine is supposed to
> > remain identical over multiple QEMU releases. In this case xenfv (or the
> > pc machine you are using, if you are not using xenfv) has to be always
> > identical. That's why I think we need to add kvmapic back to it for
> > compatibility. I know it sucks. But we can choose a different PC machine
> > with accel=xen for new VMs. 

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