Re: [Xen-devel] Hypercall privilege check

["Steven Y. Ko" <sko@xxxxxxxxxxx>]

Thanks for the quick answer!

But I guess my question was not clear enough, because I was actually
asking about Xen and how it enforces hypercalls to be made only from
ring 1. I just assumed that Xen checks if int 0x82 is executed with
the right privilege. But when I tried to find where Xen actually does
it, I couldn't find it. I hope my question is more clear this time :)

- 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!
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