Messages in this thread | | | Date | Mon, 04 Jun 2007 15:08:10 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [Xen-devel] Re: [PATCH] xen: use iret directly where possible |
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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.
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