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Date: Thu, 15 May 2014 09:23:51 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH RFC 1/8] virtio: add subsections to the
	migration stream
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To: Amit Shah <amit.shah@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, Anthony Liguori <aliguori@amazon.com>, Juan Quintela <quintela@redhat.com>, Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Andreas =?iso-8859-1?Q?F=E4rber?= <afaerber@suse.de>, Greg Kurz <gkurz@linux.vnet.ibm.com>

On Thu, May 15, 2014 at 11:34:25AM +0530, Amit Shah wrote:
> On (Wed) 14 May 2014 [17:41:38], Greg Kurz wrote:
> > There is a need to add some more fields to VirtIODevice that should be
> > migrated (broken status, endianness). The problem is that we do not
> > want to break compatibility while adding a new feature... This issue has
> > been addressed in the generic VMState code with the use of optional
> > subsections. As a *temporary* alternative to port the whole virtio
> > migration code to VMState, this patch mimics a similar subsectionning
> > ability for virtio.
> > 
> > Since each virtio device is streamed in its own section, the idea is to
> > stream subsections between the end of the device section and the start
> > of the next sections. This allows an older QEMU to complain and exit
> > when fed with subsections:
> > 
> > Unknown savevm section type 5
> > Error -22 while loading VM state
> 
> Please make this configurable -- either via configure or device
> properties.  That avoids having to break existing configurations that
> work without this patch.
> 
> > All users of virtio_load()/virtio_save() need to be patched because the
> > subsections are streamed AFTER the device itself.
> 
> Since all have the same fixup, I'm wondering if a new section can be
> added to the virtio-bus itself, which gets propagated to all devices
> upon load in the dest.
> 
> 		Amit

This calls for a way for devices to inherit properties from the bus,
which doesn't exist ATM.
Fine but let's not hold up this patchset because of this.

-- 
MST