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

From: Matthew Rosato
Subject: Re: [PATCH v3] target/s390x/kvm: Enable adapter interruption suppression again
Date: Tue, 21 Jan 2020 09:33:02 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0

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

Adding 'kvm_kernel_irqchip_required() &&' seems to do the trick;

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.

+        kvm_vm_enable_cap(s, KVM_CAP_S390_AIS, 0);
+    }
      return 0;

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