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RE: [Xen-devel] Task Switching and Xen-SVM

  • To: "Vivek Mohan" <vivek.mt@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
  • Date: Fri, 27 Oct 2006 10:59:00 +0200
  • Delivery-date: Fri, 27 Oct 2006 01:59:37 -0700
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: Acb5kXQBt+IkliCQQtW4Bv2J/9S1XgAFBCnw
  • Thread-topic: [Xen-devel] Task Switching and Xen-SVM

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Vivek Mohan
> Sent: 27 October 2006 07:29
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] Task Switching and Xen-SVM
> Hi,
> I was just going through the (svm.c) svm_vmexit_handler() in the SVM
> part of Xen, and saw that:
>          __hvm_bug(regs);
>          break;
> task switches are not intercepted and handled. I was wondering if it
> is not so, then how do you manage consistency of the VM if the Guest
> does a task switch (which could alter the value of CR3)? Am I missing
> something here?

AMD and Intel are doing the same thing here. 

Do you actually know of any (32-bit) OS that uses task switching? In my
experience, it's horribly slow (because it generally saves/restores more
of the context than is necessary). Of course, in 64-bit mode it's not
even allowed, all task-management features have been removed... I'm
saying 32-bit OS, since there's probably some 16-bit OS's out there that
do use task-switching, but those OS's are not able to run on current Xen
anyways, since Xen in it's current form is very poor at supporting
segments with base != 0 in protected mode. So far I've only seen a few
rare reports of anyone actually wishing to run a 16-bit OS... 

> Thanks,
> Vivek
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel

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