[PATCH V6 6/6] kvm: add fast mmio capabilitiy

[Jason Wang <jasowang@redhat.com>]

Cc: Gleb Natapov <gleb@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 Documentation/virtual/kvm/api.txt | 7 ++++++-
 include/uapi/linux/kvm.h          | 1 +
 virt/kvm/kvm_main.c               | 1 +
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index d9eccee..26661ef 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -1598,7 +1598,7 @@ 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];
@@ -1621,6 +1621,11 @@ 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_FAST_MMIO, a zero length mmio eventfd is allowed for
+kernel to ignore the length of guest write and get a possible faster
+response. Note the speedup may only work on some specific
+architectures and setups. Otherwise, it's as fast as wildcard mmio
+eventfd.
 
 4.60 KVM_DIRTY_TLB
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index a9256f0..ad72a61 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -824,6 +824,7 @@ struct kvm_ppc_smmu_info {
 #define KVM_CAP_MULTI_ADDRESS_SPACE 118
 #define KVM_CAP_GUEST_DEBUG_HW_BPS 119
 #define KVM_CAP_GUEST_DEBUG_HW_WPS 120
+#define KVM_CAP_FAST_MMIO 121
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9af68db..645f55d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2717,6 +2717,7 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 	case KVM_CAP_IRQFD:
 	case KVM_CAP_IRQFD_RESAMPLE:
 #endif
+	case KVM_CAP_FAST_MMIO:
 	case KVM_CAP_CHECK_EXTENSION_VM:
 		return 1;
 #ifdef CONFIG_HAVE_KVM_IRQ_ROUTING
-- 
2.1.4