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>

4 files changed, 106 insertions, 15 deletions

diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
index 2316ec1aadc..a6ec206f36b 100644
--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -141,7 +141,8 @@ int kvm_vm_ioctl_register_coalesced_mmio(struct kvm *kvm,
         dev->zone = *zone;
 
         mutex_lock(&kvm->slots_lock);
-        ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, &dev->dev);
+        ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, zone->addr,
+                                      zone->size, &dev->dev);
         if (ret < 0)
                 goto out_free_dev;
         list_add_tail(&dev->list, &kvm->coalesced_zones);
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 73358d256fa..f59c1e8de7a 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -586,7 +586,8 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 
         kvm_iodevice_init(&p->dev, &ioeventfd_ops);
 
-        ret = kvm_io_bus_register_dev(kvm, bus_idx, &p->dev);
+        ret = kvm_io_bus_register_dev(kvm, bus_idx, p->addr, p->length,
+                                      &p->dev);
         if (ret < 0)
                 goto unlock_fail;
 
diff --git a/virt/kvm/ioapic.c b/virt/kvm/ioapic.c
index 8df1ca104a7..3eed61eb486 100644
--- a/virt/kvm/ioapic.c
+++ b/virt/kvm/ioapic.c
@@ -394,7 +394,8 @@ int kvm_ioapic_init(struct kvm *kvm)
         kvm_iodevice_init(&ioapic->dev, &ioapic_mmio_ops);
         ioapic->kvm = kvm;
         mutex_lock(&kvm->slots_lock);
-        ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, &ioapic->dev);
+        ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, ioapic->base_address,
+                                      IOAPIC_MEM_LENGTH, &ioapic->dev);
         mutex_unlock(&kvm->slots_lock);
         if (ret < 0) {
                 kvm->arch.vioapic = NULL;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index aefdda390f5..d9cfb782cb8 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 @@ static void kvm_io_bus_destroy(struct kvm_io_bus *bus)
         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_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 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 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx,
         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 @@ int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
 
         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;
                 }