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Subject[KVM PATCH v7.2] kvm: add iofd support
Date
[ updated with figures, graphs, performance-test-harness info ]

iofd is a mechanism to register PIO/MMIO regions to trigger an eventfd
signal when written to by a guest. Userspace can register any arbitrary
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 iofd.

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
iofd-mmio: 200100 iops, 5.00us rtt
iofd-pio: 367300 iops, 2.72us rtt
I didn't measure qemu-pio, because I have to figure out how to register a
PIO region, 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
iofd-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>
---
include/linux/kvm.h | 12 +++++
include/linux/kvm_host.h | 2 +
virt/kvm/eventfd.c | 107 ++++++++++++++++++++++++++++++++++++++++++++++
virt/kvm/kvm_main.c | 13 ++++++
4 files changed, 134 insertions(+), 0 deletions(-)
diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index dfc4bcc..99b6e45 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -292,6 +292,17 @@ struct kvm_guest_debug {
struct kvm_guest_debug_arch arch;
};

+#define KVM_IOFD_FLAG_DEASSIGN (1 << 0)
+#define KVM_IOFD_FLAG_PIO (1 << 1)
+
+struct kvm_iofd {
+ __u64 addr;
+ __u32 len;
+ __u32 fd;
+ __u32 flags;
+ __u8 pad[12];
+};
+
#define KVM_TRC_SHIFT 16
/*
* kvm trace categories
@@ -508,6 +519,7 @@ struct kvm_irqfd {
#define KVM_DEASSIGN_DEV_IRQ _IOW(KVMIO, 0x75, struct kvm_assigned_irq)
#define KVM_ASSIGN_IRQFD _IOW(KVMIO, 0x76, struct kvm_irqfd)
#define KVM_DEASSIGN_IRQFD _IOW(KVMIO, 0x77, __u32)
+#define KVM_IOFD _IOW(KVMIO, 0x78, struct kvm_iofd)

/*
* ioctls for vcpu fds
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 1acc528..d53cb70 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -529,5 +529,7 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
int kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
int kvm_deassign_irqfd(struct kvm *kvm, int fd);
void kvm_irqfd_release(struct kvm *kvm);
+int kvm_iofd(struct kvm *kvm, unsigned long addr, size_t len,
+ int fd, int flags);

#endif
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 71afd62..8b23317 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -21,12 +21,16 @@
*/

#include <linux/kvm_host.h>
+#include <linux/kvm.h>
#include <linux/workqueue.h>
#include <linux/syscalls.h>
#include <linux/wait.h>
#include <linux/poll.h>
#include <linux/file.h>
#include <linux/list.h>
+#include <linux/eventfd.h>
+
+#include "iodev.h"

/*
* --------------------------------------------------------------------
@@ -185,3 +189,106 @@ kvm_irqfd_release(struct kvm *kvm)
list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds, list)
irqfd_release(irqfd);
}
+
+/*
+ * --------------------------------------------------------------------
+ * iofd: translate a PIO/MMIO memory write to an eventfd signal.
+ *
+ * userspace can register a PIO/MMIO address with an eventfd for recieving
+ * notification when the memory has been touched.
+ * --------------------------------------------------------------------
+ */
+
+struct _iofd {
+ u64 addr;
+ size_t length;
+ struct file *file;
+ struct kvm_io_device dev;
+};
+
+static int
+iofd_in_range(struct kvm_io_device *this, gpa_t addr, int len, int is_write)
+{
+ struct _iofd *iofd = (struct _iofd *)this->private;
+
+ return ((addr >= iofd->addr && (addr < iofd->addr + iofd->length)));
+}
+
+/* writes trigger an event */
+static void
+iofd_write(struct kvm_io_device *this, gpa_t addr, int len, const void *val)
+{
+ struct _iofd *iofd = (struct _iofd *)this->private;
+
+ eventfd_signal(iofd->file, 1);
+}
+
+/* reads return all zeros */
+static void
+iofd_read(struct kvm_io_device *this, gpa_t addr, int len, void *val)
+{
+ memset(val, 0, len);
+}
+
+static void
+iofd_destructor(struct kvm_io_device *this)
+{
+ struct _iofd *iofd = (struct _iofd *)this->private;
+
+ fput(iofd->file);
+ kfree(iofd);
+}
+
+static int
+kvm_assign_iofd(struct kvm *kvm, unsigned long addr, size_t len,
+ int fd, int flags)
+{
+ int pio = flags & KVM_IOFD_FLAG_PIO;
+ struct kvm_io_bus *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+ struct _iofd *iofd;
+ struct file *file;
+
+ file = eventfd_fget(fd);
+ if (IS_ERR(file))
+ return PTR_ERR(file);
+
+ iofd = kzalloc(sizeof(*iofd), GFP_KERNEL);
+ if (!iofd) {
+ fput(file);
+ return -ENOMEM;
+ }
+
+ iofd->dev.read = iofd_read;
+ iofd->dev.write = iofd_write;
+ iofd->dev.in_range = iofd_in_range;
+ iofd->dev.destructor = iofd_destructor;
+ iofd->dev.private = iofd;
+
+ iofd->addr = addr;
+ iofd->length = len;
+ iofd->file = file;
+
+ kvm_io_bus_register_dev(bus, &iofd->dev);
+
+ printk(KERN_DEBUG "registering %s iofd at %lx of size %d\n",
+ pio ? "PIO" : "MMIO", addr, (int)len);
+
+ return 0;
+}
+
+static int
+kvm_deassign_iofd(struct kvm *kvm, unsigned long addr, size_t len,
+ int fd, int flags)
+{
+ /* FIXME: We need an io_bus_unregister() function */
+ return -EINVAL;
+}
+
+int
+kvm_iofd(struct kvm *kvm, unsigned long addr, size_t len, int fd, int flags)
+{
+ if (flags & KVM_IOFD_FLAG_DEASSIGN)
+ return kvm_deassign_iofd(kvm, addr, len, fd, flags);
+
+ return kvm_assign_iofd(kvm, addr, len, fd, flags);
+}
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7aa9f0a..a443974 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2228,6 +2228,19 @@ static long kvm_vm_ioctl(struct file *filp,
r = kvm_deassign_irqfd(kvm, data);
break;
}
+ case KVM_IOFD: {
+ struct kvm_iofd entry;
+
+ r = -EFAULT;
+ if (copy_from_user(&entry, argp, sizeof entry))
+ goto out;
+
+ r = kvm_iofd(kvm, entry.addr, entry.len, entry.fd,
+ entry.flags);
+ if (r)
+ goto out;
+ break;
+ }
default:
r = kvm_arch_vm_ioctl(filp, ioctl, arg);
}


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