linux-kernel - Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy

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Date:	Tue, 15 Sep 2015 17:07:55 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Jason Wang <jasowang@...hat.com>, gleb@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:	mst@...hat.com, cornelia.huck@...ibm.com
Subject: Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy



On 15/09/2015 08:41, Jason Wang wrote:
> +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.

I don't really like tying the capability to MMIO, especially since
zero length ioeventfd is already accepted for virtio-ccw.

What about the following?

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 7a3cb48a644d..247944071cc8 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -1627,11 +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_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.
+With KVM_CAP_IOEVENTFD_ANY_LENGTH, a zero length ioeventfd is allowed, and
+the kernel will ignore the length of guest write and get a faster vmexit.
+The speedup may only apply to specific architectures, but the ioeventfd will
+work anyway.
 
 4.60 KVM_DIRTY_TLB
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b4f6aeaf94a6..03f3618612aa 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -830,7 +830,7 @@ struct kvm_ppc_smmu_info {
 #define KVM_CAP_GUEST_DEBUG_HW_BPS 119
 #define KVM_CAP_GUEST_DEBUG_HW_WPS 120
 #define KVM_CAP_SPLIT_IRQCHIP 121
-#define KVM_CAP_FAST_MMIO 122
+#define KVM_CAP_IOEVENTFD_ANY_LENGTH 122
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 79db45336e3a..1dc8c45d2270 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -914,9 +914,7 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 		return -EINVAL;
 
 	/* ioeventfd with no length can't be combined with DATAMATCH */
-	if (!args->len &&
-	    args->flags & (KVM_IOEVENTFD_FLAG_PIO |
-			   KVM_IOEVENTFD_FLAG_DATAMATCH))
+	if (!args->len && (args->flags & KVM_IOEVENTFD_FLAG_DATAMATCH))
 		return -EINVAL;
 
 	ret = kvm_assign_ioeventfd_idx(kvm, bus_idx, args);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0780d970d087..0b48aadedcee 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2717,7 +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_IOEVENTFD_ANY_LENGTH:
 	case KVM_CAP_CHECK_EXTENSION_VM:
 		return 1;
 #ifdef CONFIG_HAVE_KVM_IRQ_ROUTING


Paolo
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