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

["Vivek Mohan" <vivek.mt@xxxxxxxxx>]

True, not really many OSes, atleast the ones that matter, use task
switching, but I was just wondering how well Xen w/ HVM would scale as
a full architectural simulator - apparently not.

Any idea why SVM (or VMX) have no support for non-intrusive stepwise
execution of guest VMs?

-Vivek

On 10/27/06, Petersson, Mats <Mats.Petersson@xxxxxxx> wrote:
> > -----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:
> >
> > case VMEXIT_TASK_SWITCH:
> >          __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...
>
> --
> Mats
> >
> > Thanks,
> > Vivek
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >
> >
> >

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