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Re: [Xen-devel] Logging External Page Writes for a Given Domain


Thanks for your response! I've been thinking more about exactly what the requirements are, and it amounts to the following.

The end goal is to know the net result of any foreign-domain page writes to all of a domain's (call it Domain P) pages. I don't actually need to know the contents of every individual foreign write to Domain P's pages per se, just which the contents of the pages that were modified by foreign domains since Domain P was last run.

Let me give an example to illustrate. Domain P is running. It is then suspended to allow another domain, Domain X, to start execution. Domain X happens to have a few pages of Domain P mapped into its address space. (This may be due to a device driver's page mapping from Domain 0, or just another Domain mapping -- I don't care why the pages are mapped). Now, during its time slice, Domain X modifies (over a period of several writes) two of Domain P's pages: page 19 and 94. Then, Domain X is suspended and Domain Y is run. Domain Y also modifies Domain P's page 94. Finally, Domain Y is suspended so Domain P can resume. It is at this point, *just before Domain P resumes*, that I would like to be able to have a list of the pages (if any) that have been written by a foreign domain (94 and 19, in this example). With that list, I would like to log the *entire contents* of those modified pages of Domain P for my later use -- I don't need to know who wrote what when, just which pages were modified so I can log their new contents.

So, in essence, *just before* every time that Domain P is allowed to resume, I want to have a list of pages that have been modified *since Domain P finished its previous time slice*, for the purpose of logging the contents of each of those changed pages.

Having said all that, what is the most straightforward way to implement this (it needn't be the most efficient, though gross inefficiency would be nice to avoid)?

I really appreciate the feedback I've received.
Thanks again,


Anthony Liguori wrote:
Scott Baker wrote:
Hello, all:

My team's goal is to be able to log all writes that are made to any memory page of a certain domain, except those writes that the domain itself makes. That is to say, if Domain 2 is the domain we want to log page writes for (where logging is capturing what was written and its location), then we want to be able to log all the writes made by any domain /except/ Domain 2 -- i.e., writes made to shared pages that belong to Domain 2.
The details here are going to make a big difference. Do you want to know the content of every write?

You'll have to modify Xen. You can probably reuse some of the shadow paging code to track the dirty mappings of foreign pages for a domain. However, this won't track the contents of the write.

If you want to do that, you're going to have to implement a large amount of emulation to track what data gets written to the page so you can emulate the writes completely. If this is an important requirement, you may wish to try to user an emulator (Bochs or qemu).


Anthony Liguori

Ideally, we would like to be able to have these writes for domain /x/ detected and trap to a process running on Domain 0, with minimal VMM modification. Preferably, the method used would only cause significant overhead when a write is made from outside domain /x/ (rather than for every write /x/ and everyone else makes), but I'm not picky about efficiency at the moment.

The rough idea we have at this point is to make the monitor process on Domain 0 mark all the pages of Domain /x/ as read-only (while remembering which are actually read-only). Then, when a write-fault occurred, the VMM would pass it on to the monitoring process, which would then let Domain /x/ finish the write, the monitor would record what was written, and let everything continue as normal.

Unfortunately, I'm not sure how that vague sketch fits into Xen. For the page table read-only flag setting, would we use the update_va_mapping() hypercall? And, how would the monitoring process let Domain /x/ finish the write and then get control back? (Or, does the VMM know what is about to be written, so we could just pass that to the monitoring process?) Finally, where in Xen's code would we have to go to modify the fault-handling behavior so the callback could be made?

Hope I'm clear, and hope you can help a newbie!


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