libvirt users - Apr 2020 - domain: how long is new xml in saved file

Hi administrator,
	I am a cloud compute developer. I need some help from you about libvirt.
	I have a work to modify a image file which is saved by virDomainSave() or
virDomainSaveFlags(). So virDomainSaveImageGetXMLDesc() and
virDomainSaveImageDefineXML() are APIs I choosed to do.
	Because I found a sentence: A save file can be inspected or modified slightly
with virDomainSaveImageGetXMLDesc() and virDomainSaveImageDefineXML().
	But an error is happened when I do like that.  libvirt: QEMU Driver
error: operation failed: new xml too large to fit in file.
	
	I found that if I increase strlen(xml_old) to strlen(xml_new).
	if (strlen(xml_new) - strlen(xml_old) <= 29) {
		this is right;
	}
	but if (strlen(xml_new) - strlen(xml_old) &gt;= 50) {
		this is error;
	}
	But I don't choose to find an accurate number. &nbsp;I think this value
will be affected by some factors.
	For example: memory alignment, range safety or other rules.


	++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
	+ &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
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&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
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&nbsp;&nbsp;&nbsp;&nbsp;strlen(xml_old) &nbsp; &nbsp;
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&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp;+ &nbsp;free space &nbsp;+
	++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
	+ &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp;&nbsp;strlen(xml_new) &nbsp; &nbsp;
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&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
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&nbsp; &nbsp; &nbsp; &nbsp; +
	++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
	I really want to know how long free space I can use. Can you convert slightly
to a precise number?
	Thank you for taking so long to read my Email.
	Looking forward to your reply.
	
	ps:
		OS: CentOS7.4
		libvirt: 4.5.0
		hypervisor: KVM
	
	Sincerely,&nbsp;
	Vincent Wu

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.

But, what you can do is provide the changed XML not in 
virDomainSaveImageDefineXML() but provide it to virDomainRestoreFlags() 
which doesn't check for XML length.

Michal

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.
> But, what you can do is provide the changed XML not in
> virDomainSaveImageDefineXML() but provide it to virDomainRestoreFlags()
> which doesn't check for XML length.
> 
> Michal
> 
Regards,
Daniel
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