Re: [PATCH v3] target/s390x/kvm: Enable adapter interruption suppression again

[Matthew Rosato]

On 1/21/20 10:22 AM, Thomas Huth wrote:
> On 21/01/2020 15.46, Cornelia Huck wrote:
> > On Tue, 21 Jan 2020 09:33:02 -0500
> > Matthew Rosato <address@hidden> wrote:
> >
> > > On 1/20/20 12:27 PM, Cornelia Huck wrote:
> > > > On Mon, 20 Jan 2020 14:24:41 +0100
> > > > Thomas Huth <address@hidden> wrote:
> > > >    
> > > > > The AIS feature has been disabled late in the v2.10 development cycle since
> > > > > there were some issues with migration (see commit 3f2d07b3b01ea61126b -
> > > > > "s390x/ais: for 2.10 stable: disable ais facility"). We originally wanted
> > > > > to enable it again for newer machine types, but apparently we forgot to do
> > > > > this so far. Let's do it for the new s390-ccw-virtio-5.0 machine now.
> > > > >
> > > > > While at it, also add a more verbose comment why we need the *_allowed()
> > > > > wrappers in s390-virtio-ccw.c.
> > > > >
> > > > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1756946
> > > > > Reviewed-by: David Hildenbrand <address@hidden>
> > > > > Signed-off-by: Thomas Huth <address@hidden>
> > > > > ---
> > > > >    v3: Moved "s390mc->kvm_ais_allowed = false" to the end of the function
> > > > >
> > > > >    hw/s390x/s390-virtio-ccw.c         | 20 +++++++++++++++++---
> > > > >    include/hw/s390x/s390-virtio-ccw.h |  3 +++
> > > > >    target/s390x/kvm.c                 |  9 ++++++---
> > > > >    3 files changed, 26 insertions(+), 6 deletions(-)
> > > >    
> > > > > diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
> > > > > index 15260aeb9a..cf4fb4f2d9 100644
> > > > > --- a/target/s390x/kvm.c
> > > > > +++ b/target/s390x/kvm.c
> > > > > @@ -365,10 +365,13 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
> > > > >        /*
> > > > >         * The migration interface for ais was introduced with kernel 4.13
> > > > >         * but the capability itself had been active since 4.12. As migration
> > > > > -     * support is considered necessary let's disable ais in the 2.10
> > > > > -     * machine.
> > > > > +     * support is considered necessary, we only try to enable this for
> > > > > +     * newer machine types if KVM_CAP_S390_AIS_MIGRATION is available.
> > > > >         */
> > > > > -    /* kvm_vm_enable_cap(s, KVM_CAP_S390_AIS, 0); */
> > > > > +    if (kvm_ais_allowed() &&
> > > > > +        kvm_check_extension(s, KVM_CAP_S390_AIS_MIGRATION)) {
> > > >
> > > > Hnm, we actually need a kernel irqchip with the kvm flic to get ais to
> > > > work; else we'll fail with
> > > >
> > > > qemu-system-s390x: Failed to inject airq with AIS supported
> > > >
> > > > in the kernel_irqchip=off case, as we won't have an I/O adapter
> > > > registered.
> > > >
> > > > Adding 'kvm_kernel_irqchip_required() &&' seems to do the trick;
> > > > comments?
> > > >    
> > >
> > > In spirit, I agree with this idea.  But, a quick test shows that putting
> > > this check here results in ais=off for the 'none' machine case (libvirt
> > > capabilities detection).  I think we have to only look at
> > > kvm_kernel_irqchip_required() when working with a real machine.
> >
> > Sigh, I think you're right again. We need to check for the 'none'
> > machine here; but I can't think of a non-ugly way to do so...
>
> I think it might work when using kvm_kernel_irqchip_allowed() instead of
> kvm_kernel_irqchip_required() ... Matthew, could you please give it a
> try with this patch on top of mine:

Sure.

Libvirt detection works with this patch.

Alternatively, if I run qemu with kernel_irqchip=off and ais=true, I get:
qemu-system-s390x: Some features requested in the CPU model are not 
available in the configuration: ais

Which was the same result as Connie's proposal.

It reads a bit odd to me at first, but looking at the code quick I think 
this is the right answer - kvm_kernel_irqchip_allowed() will only return 
false when kernel_irqchip has been forced off as above, whereas 
kernel_irqchip_required will also return false in the case where no 
setting was specified (this is what tripped libvirt up).

Looks good to me, thanks Thomas.