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

[Scott Baker <YoshiHQ@xxxxxxxx>]

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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