Re: [Xen-devel] Re: [PATCH] xen: use iret directly where possible

[Jeremy Fitzhardinge <jeremy@goop.org>]

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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