domain: how long is new xml in saved file

[Daniel P. Berrangé]

On Mon, Apr 27, 2020 at 10:17:52AM +0200, Andrea Bolognani wrote:
> On Fri, 2020-04-24 at 10:20 -0500, Eric Blake wrote:
> > On 4/24/20 7:37 AM, Daniel P. Berrangé wrote:
> > > On Fri, Apr 24, 2020 at 02:33:13PM +0200, Michal Privoznik wrote:
> > > > On 4/24/20 6:38 AM, Vincent Wu wrote:
> > > > 
> > > > The save format is fragile. At the beginning there is a header which
> > > > describes the file, then there is libvirt section (which contains the domain
> > > > XML and a cookie) and then there is QEMU section (where QEMU saved the guest
> > > > memory). Because of this, we have to have the check you are hitting in place
> > > > so that we don't accidentally overwrite the QEMU section.
> > > 
> > > BTW, does anyone recall why we were so restrictive on the XML length
> > > in the first place ?  I looked at history and didn't see why we did
> > > it this way.
> > > 
> > > It occurrs to me that given guest typical RAM sizes measuring many
> > > 100's of MB, we could easily make the header section have 1 MB of
> > > padding, and thus allow essentially arbitrary XML updates without
> > > worry about hitting a size limit.
> > 
> > We've had guest XML reaching 1M before, but I agree that the initial 
> > saved image creation should include padding to a nice boundary to make 
> > future edits less likely to overflow the reserved heading.
> > 
> > On new enough Linux, some file systems support 
> > fallocate(FALLOC_FL_INSERT_RANGE) which can splice in a hole (all later 
> > file contents are shifted in offsets); maybe our save code could take 
> > advantage of that to repair existing saved images with insufficient 
> > header size in a more efficient manner than manually shifting the rest 
> > of the file contents ourselves.
> 
> There's a bug filed for this:
> 
>   https://bugzilla.redhat.com/show_bug.cgi?id=1229255
> 
> Both you and Dan commented on it at some point, but I thought I'd
> bring it up in case you forgot - it was a while ago :)

Hmm, I was wondering where the "pad = 1024" line referenced in that
bz comment #3 went to, and I found it was removed in

commit 6b9b21db7079888a05d192b079e68290bdf14a76
Author: Peter Krempa <pkrempa at redhat.com>
Date:   Wed Feb 17 13:10:11 2016 +0100

    qemu: Remove unnecessary calculations in qemuDomainSaveMemory
    
    Now that the file migration doesn't require us to use 'dd' and other
    legacy stuff for too old qemus we don't even have to calcuate the
    offsets and other stuff.

So ever since then, we appear to have had ZERO padding present at all.

LOoking at the saved state image for a VM appears to confirm this

At the end of the guest XML, we have a single NUL, then the cookie
XML, single NUL and then the QEMU stream marker

  <seclabel type='dynamic' model='selinux' relabel='yes'/>
</domain>
^@<cookie>
  <slirpHelper/>
</cookie>
^@QEVM^@


So it is no wonder it isn't possible to edit the image to make it longer

Regards,
Daniel
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