lkml.org 
[lkml]   [2007]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [Xen-devel] Re: [PATCH] xen: use iret directly where possible
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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-06-05 00:11    [W:0.033 / U:1.992 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site