Re: [Xen-devel] progress and a problem

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

If Xen interrupted a ring-1 activation then the hypervisor_callback
stack frame will be pushed onto the stack that was interrupted.

If Xen interrupted rings 2 or 3 then it uses SS:ESP specified in your
call to HYPERVISOR_stack_switch when creating the hypervisor_callback
stack frame.

The code that creates the callback stack frame and jumps to your
handler is in xen/arch/x86/entry.S

 -- Keir

> 
> I've got xen calling into my hypervisor_callback but i'm seeing a very 
> weird problem. On the first instruction, which is a PUSHL %eax, I take a 
> page fault. The values of %ss and %esp on stack make no sense, and in 
> fact change from run to run of the guest OS. All other values stay the 
> same. PC does look right. 
> 
> If i just put an IRETL as the first instruction of the hypervisor_callback
> all is well -- no problem. Does Xen pass in a read-only stack segment to
> the callback? I don't see similar problems on the Linux side.
> 
> If I get a stack dump, everything EXCEPT ss/esp look fine -- cs, ds, etc. 
> are all right. 
> 
> I have done a HYPERVISOR_stack_switch call, and all other interrupts work 
> save this one.
> 
> what function in Xen calls the hypervisor_callback in the guest?
> 
> ron
> 
> 
> 
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