Re: [Xen-devel] page table question!

["pradeep singh rautela" <rautelap@xxxxxxxxx>]

On 6/14/07, Jun Koi <junkoi2004@xxxxxxxxx> wrote:
> Yesterday I committed a silly mistake, and then I went on hanging
> myself as I said in my last mail. I went to heaven, and fortunately
> they have internet/email here. Indeed!
>
> The God asks me to thank people like Mats, Mark, Anthony Liguori,...
> who are always willing to answer technical questions on the list. It
> is believed that not everybody can explain problems in clear and
> easy-to-understand way like you can do. Your posts are always very
> much useful, and I believe that those helps can raise and attract a
> lot more developers for this project.
>
> You dont know how much the community appreciate your helps! Thanks god
> we have people like you here!
>
> Keep posting, and you will all go to heaven :-)
Actually this keep posting ,and you will go to heaven part is luring
enough :-).Even for people like me who want to contribute and now one
more reason to stick to Xen;).

Good one Jun.
(I hope they'll let my message in)
Regards
--psr

> >From heaven, with respects,
> Jun
>
>
> On 6/14/07, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:
> > > > "Writable pagetables" is an interface introduced for Xen 3 PV
> > > > guests to update
> > > > their pagetables.  Previous versions of Xen required guests
> > > > to be modified to
> > > > make every update to their pagetables explicitly call into Xen (and
> > > > explicitly batch those operations).  Writeable pagetables
> > > > replace this by
> > > > allowing updates to the pagetables to be implemented by the
> > > > guest as attempts
> > > > to write directly to them.  This isn't a trusted operation,
> > > > however, since
> > > > Xen uses page protections to prevent unvalidated writes going
> > > > through...
> > > > Right now this is implemented by trapping each attempt to
> > > > write to the lower
> > > > levels of the pagetables and emulating it in Xen.  Previous
> > > > implementations
> > > > were a bit more complicated but found to be unnecessary.
> > >
> > > Thanks for clarifying. I seem to remember reading about a mode where the
> > > page-table isn't "read-only" - did I just imagine that, or what?
> >
> > As far as I know normal Xen doesn't support any mode where the guest can write
> > directly to real pagetables without validation...
> >
> > I can think of two things you might have read about:
> >
> > 1) on intercepting an attempted write to an L1 pagetable, the wrpt code would
> > unhook that from the pagetable hierarchy, and then make it writable.
> > Subsequently, the guest could write to it as a normal page.  A fault would
> > occur if the CPU tried to translate a VA covered by this pagetable, at which
> > point Xen would revalidate that pagetable for safety, then hook it back into
> > the hierarchy.  This let batching of pagetable updates be entirely implicit,
> > but it later turned out that it performed less well than emulating each
> > write, and using explicit batching in the guest.
> >
> > 2) if you compile Xen with the right options, a dom0 kernel (possibly with the
> > right bumf compiled in) can run in ring0 and sort-of bypass a load of Xen's
> > normal checks.  The intent was to use this to make a kernel bootable both on
> > native hardware (eventually using a "minixen" rather than heavyweight Xen)
> > and paravirt.  This hasn't been investigated much, since Linux decided to
> > pursue the paravirt-ops solution, which allows a kernel to boot native or
> > paravirt anyway.
> >
> >
> > > > One thing I've never been clear on for shadow mode is how
> > > > accessed / dirty
> > > > bits get propagated to the guest pagetable from the shadow.
> > >
> > > Good question. I have a feeling that the answer is "it doesn't". HAP
> > > would probably solve this problem.
> >
> > Don't guests need the dirty bits for their memory management (e.g. mmap) to
> > work correctly?  Maybe one could do something scary like trapping reads to
> > guest PTEs, enabling Xen to refer to the real PTE...  Sounds a bit gross
> > though.
> >
> > HAP is definitely HAPpier :-)
> >
> > Cheers,
> > Mark
> >
> > --
> > 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!
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel

--
---
pradeep singh rautela

"Genius is 1% inspiration, and 99% perspiration" - not me :)

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel