Take a look at tools/libxc/xc_linux_save.c. It does most of what you're
looking for but in a different context (live migration of a domain).
There is a paper, IIRC, that describes how migration in Xen works too
that would be a good starter for understanding what that code's doing.
Regards,
Anthony Liguori
Scott Baker wrote:
Hello, again!
I'm back, with another question or two regarding logging external page
writes. I was hoping that someone could point me to the proper source
files where I might make the following modifications:
1. Enable dirty page tracking (in shadow paging mode), so I can know
which domain pages were modified.
2. Reset dirty pages /right after/ the domain being logged is
stopped (before any other domain runs), so that I know which pages are
dirtied after the domain stops running.
3. Dump (record) the contents of all dirty pages /right before/ the
domain being logged is about to start again.
I've been poking around in the code base, but it's a little
overwhelming. Another pointer in the right direction (e.g.,
appropriate source files/locations, etc.) would be *very* helpful.
Also, a pointer to any general documentation on how Xen's shadow
paging and dirty page tracking work would be helpful as well.
Oh, one last thing -- could these changes be made successfully on
versions of Xen >= 2.0; if not, what's the minimum/maximum version?
Thank you all,
Scott
P.S. The below is included to ensure the context of the request is
understood.
Anthony Liguori wrote:
Scott Baker wrote:
Anthony,
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.
Are you ignoring the SMP case (where two domains may be executing
simultaneously)? If so, I think the most direct approach would be to
enable the dirty page tracking in shadow paging mode, and then dump
any share pages before world switching that are dirty. It still
requires hypervisor modification, but it's reasonably straight forward.
Regards,
Anthony Liguori
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,
Scott
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).
Regards,
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!
Thanks,
Scott
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