KVM: support any-length wildcard ioeventfd

It is sometimes benefitial to ignore IO size, and only match on address.
In hindsight this would have been a better default than matching length
when KVM_IOEVENTFD_FLAG_DATAMATCH is not set, In particular, this kind
of access can be optimized on VMX: there no need to do page lookups.
This can currently be done with many ioeventfds but in a suboptimal way.

However we can't change kernel/userspace ABI without risk of breaking
some applications.
Use len = 0 to mean "ignore length for matching" in a more optimal way.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8b8fc0b792ba..bc4aaf68190c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2644,6 +2644,7 @@ int kvm_dev_ioctl_check_extension(long ext)
         case KVM_CAP_IRQ_INJECT_STATUS:
         case KVM_CAP_IRQFD:
         case KVM_CAP_IOEVENTFD:
+        case KVM_CAP_IOEVENTFD_NO_LENGTH:
         case KVM_CAP_PIT2:
         case KVM_CAP_PIT_STATE2:
         case KVM_CAP_SET_IDENTITY_MAP_ADDR: