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authorGregory Haskins <ghaskins@novell.com>2009-07-07 17:08:49 -0400
committerAvi Kivity <avi@redhat.com>2009-09-10 08:33:12 +0300
commitd34e6b175e61821026893ec5298cc8e7558df43a (patch)
tree8f2934bb0df05d18372509f9ac59aecee5884997 /virt/kvm
parent090b7aff27120cdae76a346a70db394844fea598 (diff)
KVM: add ioeventfd support
ioeventfd is a mechanism to register PIO/MMIO regions to trigger an eventfd signal when written to by a guest. Host userspace can register any arbitrary IO address with a corresponding eventfd and then pass the eventfd to a specific end-point of interest for handling. Normal IO requires a blocking round-trip since the operation may cause side-effects in the emulated model or may return data to the caller. Therefore, an IO in KVM traps from the guest to the host, causes a VMX/SVM "heavy-weight" exit back to userspace, and is ultimately serviced by qemu's device model synchronously before returning control back to the vcpu. However, there is a subclass of IO which acts purely as a trigger for other IO (such as to kick off an out-of-band DMA request, etc). For these patterns, the synchronous call is particularly expensive since we really only want to simply get our notification transmitted asychronously and return as quickly as possible. All the sychronous infrastructure to ensure proper data-dependencies are met in the normal IO case are just unecessary overhead for signalling. This adds additional computational load on the system, as well as latency to the signalling path. Therefore, we provide a mechanism for registration of an in-kernel trigger point that allows the VCPU to only require a very brief, lightweight exit just long enough to signal an eventfd. This also means that any clients compatible with the eventfd interface (which includes userspace and kernelspace equally well) can now register to be notified. The end result should be a more flexible and higher performance notification API for the backend KVM hypervisor and perhipheral components. To test this theory, we built a test-harness called "doorbell". This module has a function called "doorbell_ring()" which simply increments a counter for each time the doorbell is signaled. It supports signalling from either an eventfd, or an ioctl(). We then wired up two paths to the doorbell: One via QEMU via a registered io region and through the doorbell ioctl(). The other is direct via ioeventfd. You can download this test harness here: ftp://ftp.novell.com/dev/ghaskins/doorbell.tar.bz2 The measured results are as follows: qemu-mmio: 110000 iops, 9.09us rtt ioeventfd-mmio: 200100 iops, 5.00us rtt ioeventfd-pio: 367300 iops, 2.72us rtt I didn't measure qemu-pio, because I have to figure out how to register a PIO region with qemu's device model, and I got lazy. However, for now we can extrapolate based on the data from the NULLIO runs of +2.56us for MMIO, and -350ns for HC, we get: qemu-pio: 153139 iops, 6.53us rtt ioeventfd-hc: 412585 iops, 2.37us rtt these are just for fun, for now, until I can gather more data. Here is a graph for your convenience: http://developer.novell.com/wiki/images/7/76/Iofd-chart.png The conclusion to draw is that we save about 4us by skipping the userspace hop. -------------------- Signed-off-by: Gregory Haskins <ghaskins@novell.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Avi Kivity <avi@redhat.com>
Diffstat (limited to 'virt/kvm')
-rw-r--r--virt/kvm/eventfd.c251
-rw-r--r--virt/kvm/kvm_main.c11
2 files changed, 260 insertions, 2 deletions
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 4092b8dcd51..99017e8a92a 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -21,6 +21,7 @@
*/
#include <linux/kvm_host.h>
+#include <linux/kvm.h>
#include <linux/workqueue.h>
#include <linux/syscalls.h>
#include <linux/wait.h>
@@ -28,6 +29,9 @@
#include <linux/file.h>
#include <linux/list.h>
#include <linux/eventfd.h>
+#include <linux/kernel.h>
+
+#include "iodev.h"
/*
* --------------------------------------------------------------------
@@ -234,10 +238,11 @@ fail:
}
void
-kvm_irqfd_init(struct kvm *kvm)
+kvm_eventfd_init(struct kvm *kvm)
{
spin_lock_init(&kvm->irqfds.lock);
INIT_LIST_HEAD(&kvm->irqfds.items);
+ INIT_LIST_HEAD(&kvm->ioeventfds);
}
/*
@@ -327,3 +332,247 @@ static void __exit irqfd_module_exit(void)
module_init(irqfd_module_init);
module_exit(irqfd_module_exit);
+
+/*
+ * --------------------------------------------------------------------
+ * ioeventfd: translate a PIO/MMIO memory write to an eventfd signal.
+ *
+ * userspace can register a PIO/MMIO address with an eventfd for receiving
+ * notification when the memory has been touched.
+ * --------------------------------------------------------------------
+ */
+
+struct _ioeventfd {
+ struct list_head list;
+ u64 addr;
+ int length;
+ struct eventfd_ctx *eventfd;
+ u64 datamatch;
+ struct kvm_io_device dev;
+ bool wildcard;
+};
+
+static inline struct _ioeventfd *
+to_ioeventfd(struct kvm_io_device *dev)
+{
+ return container_of(dev, struct _ioeventfd, dev);
+}
+
+static void
+ioeventfd_release(struct _ioeventfd *p)
+{
+ eventfd_ctx_put(p->eventfd);
+ list_del(&p->list);
+ kfree(p);
+}
+
+static bool
+ioeventfd_in_range(struct _ioeventfd *p, gpa_t addr, int len, const void *val)
+{
+ u64 _val;
+
+ if (!(addr == p->addr && len == p->length))
+ /* address-range must be precise for a hit */
+ return false;
+
+ if (p->wildcard)
+ /* all else equal, wildcard is always a hit */
+ return true;
+
+ /* otherwise, we have to actually compare the data */
+
+ BUG_ON(!IS_ALIGNED((unsigned long)val, len));
+
+ switch (len) {
+ case 1:
+ _val = *(u8 *)val;
+ break;
+ case 2:
+ _val = *(u16 *)val;
+ break;
+ case 4:
+ _val = *(u32 *)val;
+ break;
+ case 8:
+ _val = *(u64 *)val;
+ break;
+ default:
+ return false;
+ }
+
+ return _val == p->datamatch ? true : false;
+}
+
+/* MMIO/PIO writes trigger an event if the addr/val match */
+static int
+ioeventfd_write(struct kvm_io_device *this, gpa_t addr, int len,
+ const void *val)
+{
+ struct _ioeventfd *p = to_ioeventfd(this);
+
+ if (!ioeventfd_in_range(p, addr, len, val))
+ return -EOPNOTSUPP;
+
+ eventfd_signal(p->eventfd, 1);
+ return 0;
+}
+
+/*
+ * This function is called as KVM is completely shutting down. We do not
+ * need to worry about locking just nuke anything we have as quickly as possible
+ */
+static void
+ioeventfd_destructor(struct kvm_io_device *this)
+{
+ struct _ioeventfd *p = to_ioeventfd(this);
+
+ ioeventfd_release(p);
+}
+
+static const struct kvm_io_device_ops ioeventfd_ops = {
+ .write = ioeventfd_write,
+ .destructor = ioeventfd_destructor,
+};
+
+/* assumes kvm->slots_lock held */
+static bool
+ioeventfd_check_collision(struct kvm *kvm, struct _ioeventfd *p)
+{
+ struct _ioeventfd *_p;
+
+ list_for_each_entry(_p, &kvm->ioeventfds, list)
+ if (_p->addr == p->addr && _p->length == p->length &&
+ (_p->wildcard || p->wildcard ||
+ _p->datamatch == p->datamatch))
+ return true;
+
+ return false;
+}
+
+static int
+kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
+{
+ int pio = args->flags & KVM_IOEVENTFD_FLAG_PIO;
+ struct kvm_io_bus *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+ struct _ioeventfd *p;
+ struct eventfd_ctx *eventfd;
+ int ret;
+
+ /* must be natural-word sized */
+ switch (args->len) {
+ case 1:
+ case 2:
+ case 4:
+ case 8:
+ break;
+ default:
+ return -EINVAL;
+ }
+
+ /* check for range overflow */
+ if (args->addr + args->len < args->addr)
+ return -EINVAL;
+
+ /* check for extra flags that we don't understand */
+ if (args->flags & ~KVM_IOEVENTFD_VALID_FLAG_MASK)
+ return -EINVAL;
+
+ eventfd = eventfd_ctx_fdget(args->fd);
+ if (IS_ERR(eventfd))
+ return PTR_ERR(eventfd);
+
+ p = kzalloc(sizeof(*p), GFP_KERNEL);
+ if (!p) {
+ ret = -ENOMEM;
+ goto fail;
+ }
+
+ INIT_LIST_HEAD(&p->list);
+ p->addr = args->addr;
+ p->length = args->len;
+ p->eventfd = eventfd;
+
+ /* The datamatch feature is optional, otherwise this is a wildcard */
+ if (args->flags & KVM_IOEVENTFD_FLAG_DATAMATCH)
+ p->datamatch = args->datamatch;
+ else
+ p->wildcard = true;
+
+ down_write(&kvm->slots_lock);
+
+ /* Verify that there isnt a match already */
+ if (ioeventfd_check_collision(kvm, p)) {
+ ret = -EEXIST;
+ goto unlock_fail;
+ }
+
+ kvm_iodevice_init(&p->dev, &ioeventfd_ops);
+
+ ret = __kvm_io_bus_register_dev(bus, &p->dev);
+ if (ret < 0)
+ goto unlock_fail;
+
+ list_add_tail(&p->list, &kvm->ioeventfds);
+
+ up_write(&kvm->slots_lock);
+
+ return 0;
+
+unlock_fail:
+ up_write(&kvm->slots_lock);
+
+fail:
+ kfree(p);
+ eventfd_ctx_put(eventfd);
+
+ return ret;
+}
+
+static int
+kvm_deassign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
+{
+ int pio = args->flags & KVM_IOEVENTFD_FLAG_PIO;
+ struct kvm_io_bus *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+ struct _ioeventfd *p, *tmp;
+ struct eventfd_ctx *eventfd;
+ int ret = -ENOENT;
+
+ eventfd = eventfd_ctx_fdget(args->fd);
+ if (IS_ERR(eventfd))
+ return PTR_ERR(eventfd);
+
+ down_write(&kvm->slots_lock);
+
+ list_for_each_entry_safe(p, tmp, &kvm->ioeventfds, list) {
+ bool wildcard = !(args->flags & KVM_IOEVENTFD_FLAG_DATAMATCH);
+
+ if (p->eventfd != eventfd ||
+ p->addr != args->addr ||
+ p->length != args->len ||
+ p->wildcard != wildcard)
+ continue;
+
+ if (!p->wildcard && p->datamatch != args->datamatch)
+ continue;
+
+ __kvm_io_bus_unregister_dev(bus, &p->dev);
+ ioeventfd_release(p);
+ ret = 0;
+ break;
+ }
+
+ up_write(&kvm->slots_lock);
+
+ eventfd_ctx_put(eventfd);
+
+ return ret;
+}
+
+int
+kvm_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
+{
+ if (args->flags & KVM_IOEVENTFD_FLAG_DEASSIGN)
+ return kvm_deassign_ioeventfd(kvm, args);
+
+ return kvm_assign_ioeventfd(kvm, args);
+}
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9c2fd025b8a..d7b9bbba26d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -979,7 +979,7 @@ static struct kvm *kvm_create_vm(void)
spin_lock_init(&kvm->mmu_lock);
spin_lock_init(&kvm->requests_lock);
kvm_io_bus_init(&kvm->pio_bus);
- kvm_irqfd_init(kvm);
+ kvm_eventfd_init(kvm);
mutex_init(&kvm->lock);
mutex_init(&kvm->irq_lock);
kvm_io_bus_init(&kvm->mmio_bus);
@@ -2271,6 +2271,15 @@ static long kvm_vm_ioctl(struct file *filp,
r = kvm_irqfd(kvm, data.fd, data.gsi, data.flags);
break;
}
+ case KVM_IOEVENTFD: {
+ struct kvm_ioeventfd data;
+
+ r = -EFAULT;
+ if (copy_from_user(&data, argp, sizeof data))
+ goto out;
+ r = kvm_ioeventfd(kvm, &data);
+ break;
+ }
#ifdef CONFIG_KVM_APIC_ARCHITECTURE
case KVM_SET_BOOT_CPU_ID:
r = 0;