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xen-devel
Re: [Xen-devel] [RFC] event channels and xen callbacks
Keir Fraser wrote:
On 4 Apr 2006, at 17:10, Mathieu Ropert wrote:
i'm currently playing a bit with event channels and hypervisor
callbacks, and i find myself stuck because i miss some informations.
Maybe you can give me some hints:
on amd64 arch, i'd like to know about hypervisor callback: ie, stack
layout when
called by Xen, stack location in memory (is it using the TSS entry as
hardware traps do?) and finally, about how HYPERVISOR_iret() expects
to find the stack upon call (like when you used iret? or something
else?).
I may write a little paper or tutorial from i've learned and didn't
found in Xen doc after i get a better view of the whole thing...
1. The stack layout on callback is just as for a native hardware
interrupt or exception. Those exceptions that push an error code when
running natively also do so when running on Xen, for example.
2. The TSS isn't fully virtualised by Xen -- instead you register your
kernel stack with the stack_switch() hypercall. Calling this is
equivalent to writing your stack pointer into your TSS when running
natively.
3. When calling HYPERVISOR_iret() there should be a iret_context
swtructure at the top of the stack. See its definition, and a big
comment, in xen/include/public/arch-x86_64.h.
-- Keir
Hi,
all those helped me alot, but i'm now getting some trouble with my
callback handler. I can't figure out why, but Xen call it with some odd
rsp (about 5 pages below the rsp i have when the callback occur). I'm
still in kernel mode when the callback happen (from what i saw, when
processing events before returning to guest after an hypercall) so my
stack isn't supposed to be changed from what i heard.
Oddly, this isn't happening with my trap handlers (i tried to trigger
some faults and dump the stack, all is fine) which are supposed to have
the same behavior.
The only difference i see is that bounce frame is created in Xen by the
syscall handler and not by the trap handler, but shouldn't be any
different, should it?
Have anyone ever got this issue, maybe i forgot to setup something prior
to enabling events...
Thanks!
Mathieu
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