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Re: [Qemu-devel] [PATCH RFC 1/1] virtio: migrate config_vector

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH RFC 1/1] virtio: migrate config_vector
Date: Wed, 13 May 2015 23:47:48 +0200

On Wed, May 13, 2015 at 08:57:00PM +0200, Christian Borntraeger wrote:
> Am 13.05.2015 um 18:14 schrieb Michael S. Tsirkin:
> >> - AFAICS, there's no easy way to add transport-specific subsections -
> >>   and simply adding config_vector in ccw would break compatibility
> > 
> > subsections break compatibility too.  The only way around that is to set
> > a flag to skip migrating config_vector for old machine types.
> My main concern is about undetected compatibility issues. A subsection will 
> tell the user that something went wrong. What happens if we just add a new
> qemu_put_byte in the stream. Will the savevm core always detect that we have
> too many or not enough bytes? If yes, adding new stuff in the stream will
> always be detected in some way as error we can go with just adding
> qemu_put_be16/qemu_get_be16 in virtio_ccw_save_config/virtio_ccw_load_config.
> Old/new QEMUs will then not be compatible - but thats probably ok as long as 
> it
> errors out.
> My understanding was that we do not have a guarentee that this will be
> detected all the time and having random junk in some variables is a debugging
> nightmare. Is that correct?
> Christian

It's not too bad - normally there's a bunch of strings that
helps you find out what's going on.
But if you really care about debuggability of migration streams, help move
forward dgilbert's RFC that switched to a self-delimiting format.
Just piling up random hacks in virtio seems like a wrong approach.


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