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>

1 files changed, 4 insertions, 2 deletions

diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index efad72385058..76e3f1cd0369 100644
--- a/arch/x86/kvm/i8254.c
+++ b/arch/x86/kvm/i8254.c
@@ -713,14 +713,16 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
         kvm_register_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
 
         kvm_iodevice_init(&pit->dev, &pit_dev_ops);
-        ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, &pit->dev);
+        ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, KVM_PIT_BASE_ADDRESS,
+                                      KVM_PIT_MEM_LENGTH, &pit->dev);
         if (ret < 0)
                 goto fail;
 
         if (flags & KVM_PIT_SPEAKER_DUMMY) {
                 kvm_iodevice_init(&pit->speaker_dev, &speaker_dev_ops);
                 ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS,
-                                                &pit->speaker_dev);
+                                              KVM_SPEAKER_BASE_ADDRESS, 4,
+                                              &pit->speaker_dev);
                 if (ret < 0)
                         goto fail_unregister;
         }