Xen newbie: inter-domain communication

Xen newbie: inter-domain communication

[Chotu Ram]

Hi All,

I have started exploring Xen couple of weeks back and have some questions 
regarding the inter-domain communication and the save/restore method used 
for hibernation of domains.

1. As far as I can understand (and I might be wrong) Xen maintains two 
domains dom0 (control domain) and dom1 (the other guest OSes). When the 
documentation says inter-domain communication, is it only referring to the 
communication between the dom0 and dom1, or it can also be applied to the 
communication among the guest OSes (unprivileged that is).

2. What are the capabilities of the inter-domain communication method. 
Specifically: what protocol is used to communicate?, what kind of 
information can be communicated? Can this communication be used for 
logging/auditing? Does anybody has any experience using this method or has 
any working example?

3. When the domain save operation is done, Xen saves the memory image to a 
file and then uses it back to restore the domain (with xm save/restore 
commands). What is the format of this image file. Can it be read somehow to 
know the status of the guest OS when it was hibernated? I am sure restore 
function will read it somehow and use it to restore the domain, but any 
direct way to understand it... like an example or something like that?

I would appreciate any help regarding these queries.

Thanks

Chotu




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Re: Xen newbie: inter-domain communication

[Derrik Pates]

Chotu Ram wrote:
> 1. As far as I can understand (and I might be wrong) Xen maintains two 
> domains dom0 (control domain) and dom1 (the other guest OSes). When the 
> documentation says inter-domain communication, is it only referring to 
> the communication between the dom0 and dom1, or it can also be applied 
> to the communication among the guest OSes (unprivileged that is).

A "domain" is a single virtual execution space, more or less. Each 
"virtual machine" is a separate domain. (When you do "xm list", the "Id" 
field is the "domain ID".) So yes, an inter-domain commmunication 
channel can exist between domain 0 and any other domain (running virtual 
machine).

> 2. What are the capabilities of the inter-domain communication method. 
> Specifically: what protocol is used to communicate?, what kind of 
> information can be communicated? Can this communication be used for 
> logging/auditing? Does anybody has any experience using this method or 
> has any working example?

As I understand, the "protocol" is basically just a per-channel 
"interrupt" (to get the other side's attention as needed), and a shared 
page, which is exchanged between the two domains as needed (only one can 
be interacting with it at a time, I think). The virtual block device 
driver and virtual network interface driver, as I understand it, 
structure it as a big ring buffer, and encode messages (and assign pages 
of their allocated memory to the other side of the channel for a 
DMA-like exchange method) in that ring buffer. Look at the developer 
documentation for a better explanation.

-- 
Derrik Pates
dpates@dsdk12.net


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Re: Xen newbie: inter-domain communication

[Mark Williamson]

> 1. As far as I can understand (and I might be wrong) Xen maintains two
> domains dom0 (control domain) and dom1 (the other guest OSes). When the
> documentation says inter-domain communication, is it only referring to the
> communication between the dom0 and dom1, or it can also be applied to the
> communication among the guest OSes (unprivileged that is).

As another poster said a "domain" is the execution context a virtual machine 
runs in, similar to how a "process" is the execution context a UNIX program 
runs in.  Each VM exists in it's own domain.

Domain 0 is a special case - it's the domain that happens to get control of 
the real physical hardware.  "Domain 1" is a colloquial term we often use to 
refer to a domain that isn't dom0 ;-)

There are various kinds of "Interdomain communication".

1) the virtual network interface
2) the virtual block interface
3) the control interface

Each of these uses an "event channel" (inter-domain interrupt) and page of 
shared memory.  Each has its own protocol.  Other types of interface will use 
their own protocol, as suited to that device.

Typically these occur between dom0 and other domains.  More complicated 
connectivity might occur, e.g. if you're using driver domains.

> 2. What are the capabilities of the inter-domain communication method.
> Specifically: what protocol is used to communicate?, 

Each interface uses a ring buffer into which small messages are inserted.  
These messages are defined as structs in the header files in 
xen/include/public/io/, which basically define much of the protocol.

You'd have to look at the source for the netif / blkif / ctrlif drivers to see 
how it all works.

For the network and block interfaces, the information in these messages is 
used to direct DMA transfers directly into / out of guest memory.

> what kind of 
> information can be communicated?

For block and net, information required to do the main IO transfer.  The 
control interface is used for generic control messaging (setting up devices, 
transferring console data, blah).

> Can this communication be used for 
> logging/auditing? Does anybody has any experience using this method or has
> any working example?

I seem to remember somebody was working on a secure logging framework.  I 
don't know how they're getting on, though.

> 3. When the domain save operation is done, Xen saves the memory image to a
> file and then uses it back to restore the domain (with xm save/restore
> commands). What is the format of this image file.

There's not an official spec, I'm afraid you'd have to look at the suspend 
code to figure out how the files are structured.

> Can it be read somehow to 
> know the status of the guest OS when it was hibernated?

Possible in principle.  One might (for instance) construct a tool that'd 
convert a suspend file into a core file, which you could then inspect using 
GDB.  Don't think this has been done, though.


> I would appreciate any help regarding these queries.

HTH,
Mark

>
> Thanks
>
> Chotu
>
>
>
>
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Re: Xen newbie: inter-domain communication

[Kip Macy]

>
> > Can it be read somehow to
> > know the status of the guest OS when it was hibernated?
>
> Possible in principle.  One might (for instance) construct a tool that'd
> convert a suspend file into a core file, which you could then inspect using
> GDB.  Don't think this has been done, though.
>

This is something I need to do for in-house purposes in any event. I'll
try to do it next week if I can get some assurance that the changes will
be promptly committed.


				-Kip


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Re: Xen newbie: inter-domain communication

[Mark Williamson]

Is it good enough for your purposes to reprocess a suspend file, or will you 
be core dumping from the domain's memory image?  I'd have thought the latter 
would better suited to debugging usage.

Mark

On Saturday 08 January 2005 03:52, Kip Macy wrote:
> > > Can it be read somehow to
> > > know the status of the guest OS when it was hibernated?
> >
> > Possible in principle.  One might (for instance) construct a tool that'd
> > convert a suspend file into a core file, which you could then inspect
> > using GDB.  Don't think this has been done, though.
>
> This is something I need to do for in-house purposes in any event. I'll
> try to do it next week if I can get some assurance that the changes will
> be promptly committed.
>
>
> 				-Kip


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Re: Xen newbie: inter-domain communication

[Kip Macy]

On Sat, 8 Jan 2005, Mark Williamson wrote:

> Is it good enough for your purposes to reprocess a suspend file, or will you
> be core dumping from the domain's memory image?  I'd have thought the latter
> would better suited to debugging usage.

Last time I looked at the suspend code it looked very linux specific.
I need something more generic. Hence I'm inclined to choose the latter.

			-Kip



>
> Mark
>
> On Saturday 08 January 2005 03:52, Kip Macy wrote:
> > > > Can it be read somehow to
> > > > know the status of the guest OS when it was hibernated?
> > >
> > > Possible in principle.  One might (for instance) construct a tool that'd
> > > convert a suspend file into a core file, which you could then inspect
> > > using GDB.  Don't think this has been done, though.
> >
> > This is something I need to do for in-house purposes in any event. I'll
> > try to do it next week if I can get some assurance that the changes will
> > be promptly committed.
> >
> >
> > 				-Kip
>
>
> -------------------------------------------------------
> The SF.Net email is sponsored by: Beat the post-holiday blues
> Get a FREE limited edition SourceForge.net t-shirt from ThinkGeek.
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> _______________________________________________
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> Xen-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/xen-devel
>


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