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

[Matthew Rosato <mjrosato@linux.ibm.com>]

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 <mjrosato@linux.ibm.com> wrote:
>>
>>> On 1/20/20 12:27 PM, Cornelia Huck wrote:
>>>> On Mon, 20 Jan 2020 14:24:41 +0100
>>>> Thomas Huth <thuth@redhat.com> 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 <david@redhat.com>
>>>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>>>>> ---
>>>>>    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.