KVM: Intelligent device lookup on I/O bus
Currently the method of dealing with an IO operation on a bus (PIO/MMIO)
is to call the read or write callback for each device registered
on the bus until we find a device which handles it.
Since the number of devices on a bus can be significant due to ioeventfds
and coalesced MMIO zones, this leads to a lot of overhead on each IO
operation.
Instead of registering devices, we now register ranges which points to
a device. Lookup is done using an efficient bsearch instead of a linear
search.
Performance test was conducted by comparing exit count per second with
200 ioeventfds created on one byte and the guest is trying to access a
different byte continuously (triggering usermode exits).
Before the patch the guest has achieved 259k exits per second, after the
patch the guest does 274k exits per second.
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index aefdda3..d9cfb78 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -47,6 +47,8 @@
#include <linux/srcu.h>
#include <linux/hugetlb.h>
#include <linux/slab.h>
+#include <linux/sort.h>
+#include <linux/bsearch.h>
#include <asm/processor.h>
#include <asm/io.h>
@@ -2391,24 +2393,92 @@
int i;
for (i = 0; i < bus->dev_count; i++) {
- struct kvm_io_device *pos = bus->devs[i];
+ struct kvm_io_device *pos = bus->range[i].dev;
kvm_iodevice_destructor(pos);
}
kfree(bus);
}
+int kvm_io_bus_sort_cmp(const void *p1, const void *p2)
+{
+ const struct kvm_io_range *r1 = p1;
+ const struct kvm_io_range *r2 = p2;
+
+ if (r1->addr < r2->addr)
+ return -1;
+ if (r1->addr + r1->len > r2->addr + r2->len)
+ return 1;
+ return 0;
+}
+
+int kvm_io_bus_insert_dev(struct kvm_io_bus *bus, struct kvm_io_device *dev,
+ gpa_t addr, int len)
+{
+ if (bus->dev_count == NR_IOBUS_DEVS)
+ return -ENOSPC;
+
+ bus->range[bus->dev_count++] = (struct kvm_io_range) {
+ .addr = addr,
+ .len = len,
+ .dev = dev,
+ };
+
+ sort(bus->range, bus->dev_count, sizeof(struct kvm_io_range),
+ kvm_io_bus_sort_cmp, NULL);
+
+ return 0;
+}
+
+int kvm_io_bus_get_first_dev(struct kvm_io_bus *bus,
+ gpa_t addr, int len)
+{
+ struct kvm_io_range *range, key;
+ int off;
+
+ key = (struct kvm_io_range) {
+ .addr = addr,
+ .len = len,
+ };
+
+ range = bsearch(&key, bus->range, bus->dev_count,
+ sizeof(struct kvm_io_range), kvm_io_bus_sort_cmp);
+ if (range == NULL)
+ return -ENOENT;
+
+ off = range - bus->range;
+
+ while (off > 0 && kvm_io_bus_sort_cmp(&key, &bus->range[off-1]) == 0)
+ off--;
+
+ return off;
+}
+
/* kvm_io_bus_write - called under kvm->slots_lock */
int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
int len, const void *val)
{
- int i;
+ int idx;
struct kvm_io_bus *bus;
+ struct kvm_io_range range;
+
+ range = (struct kvm_io_range) {
+ .addr = addr,
+ .len = len,
+ };
bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
- for (i = 0; i < bus->dev_count; i++)
- if (!kvm_iodevice_write(bus->devs[i], addr, len, val))
+ idx = kvm_io_bus_get_first_dev(bus, addr, len);
+ if (idx < 0)
+ return -EOPNOTSUPP;
+
+ while (idx < bus->dev_count &&
+ kvm_io_bus_sort_cmp(&range, &bus->range[idx]) == 0) {
+ if (!kvm_iodevice_write(bus->range[idx].dev, addr, len, val))
return 0;
+ idx++;
+ }
+
return -EOPNOTSUPP;
}
@@ -2416,19 +2486,33 @@
int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
int len, void *val)
{
- int i;
+ int idx;
struct kvm_io_bus *bus;
+ struct kvm_io_range range;
+
+ range = (struct kvm_io_range) {
+ .addr = addr,
+ .len = len,
+ };
bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
- for (i = 0; i < bus->dev_count; i++)
- if (!kvm_iodevice_read(bus->devs[i], addr, len, val))
+ idx = kvm_io_bus_get_first_dev(bus, addr, len);
+ if (idx < 0)
+ return -EOPNOTSUPP;
+
+ while (idx < bus->dev_count &&
+ kvm_io_bus_sort_cmp(&range, &bus->range[idx]) == 0) {
+ if (!kvm_iodevice_read(bus->range[idx].dev, addr, len, val))
return 0;
+ idx++;
+ }
+
return -EOPNOTSUPP;
}
/* Caller must hold slots_lock. */
-int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx,
- struct kvm_io_device *dev)
+int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
+ int len, struct kvm_io_device *dev)
{
struct kvm_io_bus *new_bus, *bus;
@@ -2440,7 +2524,7 @@
if (!new_bus)
return -ENOMEM;
memcpy(new_bus, bus, sizeof(struct kvm_io_bus));
- new_bus->devs[new_bus->dev_count++] = dev;
+ kvm_io_bus_insert_dev(new_bus, dev, addr, len);
rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
synchronize_srcu_expedited(&kvm->srcu);
kfree(bus);
@@ -2464,9 +2548,13 @@
r = -ENOENT;
for (i = 0; i < new_bus->dev_count; i++)
- if (new_bus->devs[i] == dev) {
+ if (new_bus->range[i].dev == dev) {
r = 0;
- new_bus->devs[i] = new_bus->devs[--new_bus->dev_count];
+ new_bus->dev_count--;
+ new_bus->range[i] = new_bus->range[new_bus->dev_count];
+ sort(new_bus->range, new_bus->dev_count,
+ sizeof(struct kvm_io_range),
+ kvm_io_bus_sort_cmp, NULL);
break;
}