Re: [Qemu-devel] drop -enable-nesting

[Daniel P. Berrange]

On Mon, May 30, 2011 at 06:19:14PM +0300, Avi Kivity wrote:
> On 05/30/2011 06:15 PM, Jan Kiszka wrote:
> >On 2011-05-30 17:10, Roedel, Joerg wrote:
> >>  On Mon, May 30, 2011 at 11:04:02AM -0400, Jan Kiszka wrote:
> >>>  On 2011-05-30 16:38, Nadav Har'El wrote:
> >>>>  On Mon, May 30, 2011, Jan Kiszka wrote about "drop -enable-nesting 
> >>>> (was: [PATCH 3/7] cpu model bug fixes and definition corrections...)":
> >>>>>  On 2011-05-30 10:18, Roedel, Joerg wrote:
> >>>>>>  On Sat, May 28, 2011 at 04:39:13AM -0400, Jan Kiszka wrote:
> >>>>>>
> >>>>>>>  J�rg, how to deal with -enable-nesting in qemu-kvm to align behavior
> >>>>>>>  with upstream?
> >>>>>>
> >>>>>>  My personal preference is to just remove it. In upstream-qemu it is
> >>>>>>  enabled/disabled by +/-svm. -enable-nesting is just a historic thing
> >>>>>>  which can be wiped out.
> >>>>
> >>>>  "-enable-nesting" could remain as a synonym for enabling either VMX or 
> >>>> SVM
> >>>>  in the guest, depending on what was available in the host (because KVM 
> >>>> now
> >>>>  supports both nested SVM and nested VMX, but not SVM-on-VMX or vice 
> >>>> versa).
> >>>
> >>>  Why? Once nesting is stable (I think SVM already is), there is no reason
> >>>  for an explicit enable. And you can always mask it out via -cpu.
> >>>
> >>>  BTW, what are the defaults for SVM right now in qemu-kvm and upstream?
> >>>  Enable if the modeled CPU supports it?
> >>
> >>  qemu-kvm still needs -enable-nesting, otherwise it is disabled. Upstream
> >>  qemu should enable it unconditionally (can be disabled with -cpu ,-svm).
> >
> >Then let's start with aligning qemu-kvm defaults to upstream? I guess
> >that's what the diff I was citing yesterday is responsible for.
> >
> >In the same run, -enable-nesting could dump a warning on the console
> >that this switch is obsolete and will be removed from future versions.
> 
> I think it's safe to drop -enable-nesting immediately.  Dan, does
> libvirt make use of it?

Yes, but it should be safe to drop it. Currently, if the user specifies
a CPU with the 'svm' flag present in libvirt guest XML, then we will
pass args '-cpu ....+svm -enable-nesting'. So if we drop --enable-nesting,
then libvirt will simply omit it and everything should still work because
we have still got +svm set.

Daniel
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