kvm: add capability for any-length ioeventfds

[firefly-linux-kernel-4.4.55.git] / Documentation / virtual / kvm / api.txt

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index e3e9c41721a2dc68588c3a6f04c7c8105f804645..34cc068e81ea38ca4cf53cc1edb99b1ec1061e83 100644 (file)
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -1604,7 +1604,7 @@ provided event instead of triggering an exit.
 struct kvm_ioeventfd {
        __u64 datamatch;
        __u64 addr;        /* legal pio/mmio address */
-       __u32 len;         /* 1, 2, 4, or 8 bytes    */
+       __u32 len;         /* 0, 1, 2, 4, or 8 bytes    */
        __s32 fd;
        __u32 flags;
        __u8  pad[36];
@@ -1627,6 +1627,10 @@ to the registered address is equal to datamatch in struct kvm_ioeventfd.
 For virtio-ccw devices, addr contains the subchannel id and datamatch the
 virtqueue index.
 
+With KVM_CAP_IOEVENTFD_ANY_LENGTH, a zero length ioeventfd is allowed, and
+the kernel will ignore the length of guest write and may get a faster vmexit.
+The speedup may only apply to specific architectures, but the ioeventfd will
+work anyway.
 
 4.60 KVM_DIRTY_TLB