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


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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