Re: [Xen-devel] Hypercall privilege check

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

>
> Great! Exactly what I was looking for. Thanks.

Cool.

> > I imagine you'd get a GPF if you tried to hypercall from userspace, but
> > I'm not so familiar with lowlevel x86 goop (at least, not when my
> > manual's in another building!) so I can't be certain.
> >
> > Trivia: As a result of hypercalls being only allowed from ring 1, the
> > Minix port (for Xen 2.0, I think) which uses ring 1, 2 and 3 has portions
> > of the kernel in ring 2 calling portions in ring 1 to actually perform
> > hypercalls.
>
> This is interesting. I think I should look at Minix port. Thanks a lot.

Are you doing an OS port by any chance?

The Minix port was done by a Masters student in Ireland.  I think he started 
on Xen 2.0 and I'm not sure he ever moved it over to Xen 3.0 after writing up 
his dissertation.

Code is probably about ... somewhere.  Google knows, I expect ;-)

Cheers,
Mark

> - Steve
>
> > Cheers,
> > Mark
> >
> > > - Steve
> > >
> > > On 6/28/07, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:
> > > > > I'm wondering where Xen actually checks whether a hypercall is made
> > > > > from ring 1 or ring 3. Could anyone point me out to the code? Any
> > > > > pointer would be much appreciated. Thanks!
> > > >
> > > > Hypercalls are made from ring 1.  The userspace tools do make
> > > > hypercalls sometimes, but they do it by calling a special kernel
> > > > driver, which then does the actual hypercall.  *actually* what that
> > > > does is to jump into the hypercall transfer page at the right
> > > > location.  This page contains the actual implementation of the
> > > > hypercall (using this indirection allows future versions of Xen to
> > > > supply different implementations of the calls, if appropriate).
> > > >
> > > > Take a look at:
> > > > drivers/xen/privcmd/privcmd.c  in the XenLinux tree.  This is used by
> > > > the dom0 tools to do control plane operations.
> > > >
> > > > Also, take a look at: include/asm-i386/mach-xen/asm/hypercall.h which
> > > > contains implementations of some other hypercalls.
> > > >
> > > > HTH,
> > > > 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
> >
> > --
> > 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!



-- 
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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