Re: [Qemu-devel] [Qemu-ppc] KVM-PR is broken with current QEMU

[Thomas Huth <thuth@redhat.com>]

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On Thu, 22 Sep 2016 07:30:52 +0200
Thomas Huth <thuth@redhat.com> wrote:

> On Thu, 22 Sep 2016 11:57:15 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Wed, Sep 21, 2016 at 10:22:11AM +0200, Thomas Huth wrote:
> > > On 20.09.2016 16:39, Cédric Le Goater wrote:
> > > > On 09/20/2016 04:24 PM, Thomas Huth wrote:
> > > >> On 20.09.2016 16:04, Cédric Le Goater wrote:
> > > [...]
> > > >>> There are other issues after in the guest (kernel crashing). But I think
> > > >>> these are related to TM which is not supported in KVM-PR. I am not sure
> > > >>> where we are on that point.
> > > >>
> > > >> There was a patch some months ago:
> > > >>
> > > >> https://lists.gnu.org/archive/html/qemu-ppc/2016-04/msg00046.html
> > > >>
> > > >> ... but I think it has never been included, as far as I can see.
> > > > 
> > > > and with that patch, the guest fully boots. But David had some concerns
> > > > on the way it is done. It would be nice to put some cycle on this. 
> > > 
> > > Looking at the mail thread, I think TM should be currently disabled for
> > > both, KVM-PR and TCG, i.e. only enabled for KVM-HV. The TM support in
> > > TCG is just fake, since TBEGIN always fails.
> > 
> > Right.  So there's two questions here
> > 
> > 1) Is qemu correctly advertising availability of TM in the device
> > tree?
> 
> If I've got that right, it's currently always advertising TM, even if
> it's not really available (in TCG mode and PR mode).
> 
> > If not we need to fix that, which might involve adding a kernel
> > capability for the PR case.
> > 
> > 2) Is the kvm unit test properly checking for availability of TM
> > before executing?
> 
> Not yet. That's why it would be good to get a proper way for testing
> for the availability of TM --> i.e. something like Anton's patch.
> 
> > > Once we've got proper TM support in TCG, this can be easily changed
> > > within QEMU. And once we've got TM support in KVM-PR, I think we should
> > > also introduce a capability flag to KVM which can be used to inform QEMU
> > > about this.
> > > 
> > > So I think Anton's patch currently just lacks the check for TCG.
> > > Anton, if you've got some spare minutes, could you maybe send an updated
> > > version of that patch?
> > 
> > Sorry, which patch of Anton's?
> 
> This one:
> https://lists.gnu.org/archive/html/qemu-ppc/2016-04/msg00415.html

Actually, looking at that whole pa-feature code in QEMU, I think
there's some more work to do here: Everything that is not using
mmu_model==POWERPC_MMU_2_06 is automatically getting pa_features_207.
This is sometimes completely wrong, for example when running with
KVM-PR, the mmu_model for POWER7 is POWERPC_MMU_2_06a instead.
Or when running with TCG, I think it's also perfectly legal to run the
pseries machine with a POWER5+ or PPC970 CPU - and we certainly do not
want to use pa_features_207 there.

So if you like, I can try to come up with a small patch series that
cleans up this mess - and I could also include an updated versions of
Anton's patch there unless he wants to redo the changes on his own...?

 Thomas

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