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, 2 insertions, 1 deletions

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index a8f4ee5d2e82..39098a61f41c 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -529,7 +529,7 @@ enum {
 struct kvm_ioeventfd {
         __u64 datamatch;
         __u64 addr;        /* legal pio/mmio address */
-        __u32 len;         /* 1, 2, 4, or 8 bytes    */
+        __u32 len;         /* 1, 2, 4, or 8 bytes; or 0 to ignore length */
         __s32 fd;
         __u32 flags;
         __u8  pad[36];
@@ -743,6 +743,7 @@ struct kvm_ppc_smmu_info {
 #define KVM_CAP_IOAPIC_POLARITY_IGNORED 97
 #define KVM_CAP_ENABLE_CAP_VM 98
 #define KVM_CAP_S390_IRQCHIP 99
+#define KVM_CAP_IOEVENTFD_NO_LENGTH 100
 
 #ifdef KVM_CAP_IRQ_ROUTING