Andi Kleen wrote:
> Ah I assumed the hypervisor would just check IF in ring 1 too.
> It would certainly make this easier, but then the additional trap
> of setting it would be also somewhat expensive agreed.
>
Xen doesn't do that because, while it could track sti/cli (expensively),
iret and popf quietly ignore the IF state in ring 1, and so there's lots
of scope for interrupt state getting lost.
> I must say I still hate the patch; it has all the signs of something that
> will be very nasty to maintain later.
>
Well, the corresponding xen-unstable code has been a bit of a trial to
maintain. I made this as simple and self-contained as possible (with
very little non-locality) to try and keep it maintainable.
I agree its all a bit subtle, but in its favour:
1. It's internal to the implementation of the iret pvop, which does
have a fairly well-defined and stable interface (same as iret
instruction, essentially)
2. Comments!
3. Relatively simple implementation (only one register to deal with
in the slow-path handler, for example)
The annoying non-local thing is the test in the xen upcall handler, but
that's unavoidable.
J
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