Re: [Qemu-devel] [RFC PATCH 4/4] qemu-options: Do not show -enable-kvm and -enable-hax in the docs anymore

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jun 26, 2018 at 01:06:23PM -0300, Eduardo Habkost wrote:
> On Tue, Jun 26, 2018 at 03:05:33PM +0200, Paolo Bonzini wrote:
> > On 26/06/2018 14:29, Eduardo Habkost wrote:
> > > On Tue, Jun 26, 2018 at 07:57:18AM +0200, Paolo Bonzini wrote:
> > >> On 25/06/2018 21:51, Eduardo Habkost wrote:
> > >>> In either case, I'm not arguing (yet) for changing the default
> > >>> upstream.  I'm just arguing for upstream QEMU to not make any
> > >>> promises about the default.
> > >>
> > >> It would be a guest ABI breakage for TCG guests, so it would only apply
> > >> to new machine types.  I don't think it's worth the complication.
> > > 
> > > That's exactly the point: I want to stop promising a stable guest
> > > ABI when the accelerator is omitted, because I see no benefit in
> > > wasting energy on this.
> > 
> > On the other hand I see no benefit in changing a default that people are
> > obviously not using (since most people use KVM, not TCG).  Distros will
> > keep shipping, and people will keep using qemu-kvm even if we change the
> > default.
> 
> Not changing the default is different from promising we will keep
> ABI compatibility if the accelerator is omitted.  I just want to
> get rid of the latter.

I guess the key question is what is the risk of causing problems if we
switch from tcg to kvm:tcg when accelerator is omitted ?

Based on what I've seen the likely troublespot would be people who are
using QEMU inside a guest with nested-virt enabled. Some nested-virt
impls are buggy and will cause L2 guest hangs, or worse L1 host crashes.
Then again we're not causing that brokenness - just revealing what
already exists.


Regards,
Daniel
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