Re: [Xen-devel] Re: Xen/ia64 presentation

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 28 Apr 2005, at 19:58, Hollis Blanchard wrote:

> > The pointer passed to IRQ
> > handlers is a pointer to that state on the stack. If we do not pass it
> > explicitly to the handler then it is very hard to reliably recalculate
> > it if it is needed, and it is useful for debugging purposes at the 
> > very
> > least.
> What "debugging purposes"?
e.e., the 'd' key handler that dumps register state.

> We've already seen that the only common code
> using that type is to pass to (architecture-specific) show_registers().
Yes. Common code never dereferences cpu_user_regs, but it needs to 
receive it and pass it on.
> x86 can get at its state by masking off ESP. PPC can get at it via
> 'current'. Why then pass a pointer through all interrupt handlers when
> none of them care?
No, that gets you an exec_domain (or 'vcpu', as we plan to rename it). 
Or it can get you the guest's cpu_user_regs. But if you interrupted a 
Xen activation then there is no way of getting at that outer 
activation's register state.
Even Linux has a pt_regs parameter passed to every interrupt handler, 
so it must surely be a good thing to do. ;-)
 -- Keir


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