> > I actually want do to something similar, but simpler. I'm only
> > interested in keeping track of pages that a guest domain is accessing
> > (both reads and writes). I'm _not_ looking for the *exact* memory
> > address -- just the physical page being accessed. Can log dirty be
> > modified to keep track of read accesses as well?
>
> This isn't far from what I'm doing (except I need to look at one or a
> few pages, which makes life somewhat easier).
>
> You'd have to change the page-table writes so that they are written with
> "not present", and then update your statistics based on the page-fault.
> You'll have to "fix" the fault and then reset the page-table, which is
> probably easiest done by using the x86_emulate_memop() function
> [alternatively, set the trace-bit in the flags on stack before exiting
> the PF-handler, take the trace-interrupt, reset the page-table and
> continue].
>
> However, if you're doing this for every memory access of the guest,
> you'll not get much work done... :-(
As long as he just wants to keep track of "what pages did the guest access in
the last n milliseconds" (for instance) it should be OK - maintain a bitmap
(logdirty style) of accessed pages in Xen, then don't mark pages non-present
again until the next time period.
It'd still require some extension of the Xen code and would still hurt
performance but might be able to run fast enough to run a usable userspace
(maybe...? my guess. depends on the polling period of course - blowing away
the PTs too frequently would hurt a lot)
Cheers,
Mark
> --
> mats
>
> > Thanks,
> > Diwaker
> >
> > On 10/9/06, Security Initiative Team <passrete@xxxxxxxxx> wrote:
> > > My main purpose is to know when a user-level application in DomU
> > > is updating its memory.
> > > (Tracking changes to the stack segment might be too hard
> >
> > due to frequent
> >
> > > memory updates, so maybe only the "text" segment).
> > >
> > > I want to be able to track this from either Dom0 or the
> >
> > hypervisor layer,
> >
> > > whichever is easier.
> > >
> > > When is ptwr_emulated_update() used and when is do_mmu_update()
> > > used?
> > >
> > > Thanks,
> > > -Criag
> > >
> > >
> > > "Petersson, Mats" <Mats.Petersson@xxxxxxx> wrote:
> > >
> > > What do you ACTUALLY want to do?
> > >
> > > log-dirty doesn't log to a file - it keeps track of "dirty"
> >
> > pages in a list
> >
> > > in memory, but doesn't actually store it in a file [ever, at all].
> > >
> > > do_mmu_update is possibly a good place to hook into, but it
> >
> > depends on what
> >
> > > you want to do... [And it's non-trivial code, so beware of
> >
> > complications
> >
> > > from changing it].
> > >
> > > You may want to look at ptwr_emulated_update, as that's
> >
> > used when the
> >
> > > do_mmu_update() hypercall isn't used to update a page-table-entry.
> > >
> > > --
> > > Mats
> > >
> > >
> > > ________________________________
> >
> > --
> > Web/Blog/Gallery: http://floatingsun.net/blog
> >
> > _______________________________________________
> > Xen-users mailing list
> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
>
> _______________________________________________
> Xen-users mailing list
> Xen-users@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-users
--
Dave: Just a question. What use is a unicyle with no seat? And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!
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