On Mon, Mar 31, 2014 at 09:50:44PM +0300, Michael S. Tsirkin wrote: > With KVM, MMIO is much slower than PIO, due to the need to > do page walk and emulation. But with EPT, it does not have to be: we > know the address from the VMCS so if the address is unique, we can look > up the eventfd directly, bypassing emulation. > > Unfortunately, this only works if userspace does not need to match on > access length and data. The implementation adds a separate FAST_MMIO > bus internally. This serves two purposes: > - minimize overhead for old userspace that does not use eventfd with > lengtth = 0 > - minimize disruption in other code (since we don't know the length, > devices on the MMIO bus only get a valid address in write, this > way we don't need to touch all devices to teach them to handle > an invalid length) > > At the moment, this optimization only has effect for EPT on x86. > > It will be possible to speed up MMIO for NPT and MMU using the same > idea in the future. > > With this patch applied, on VMX MMIO EVENTFD is essentially as fast as PIO. > I was unable to detect any measureable slowdown to non-eventfd MMIO. > > Making MMIO faster is important for the upcoming virtio 1.0 which > includes an MMIO signalling capability. > > The idea was suggested by Peter Anvin. Lots of thanks to Gleb for > pre-review and suggestions. > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
Ping. Marcelo, any chance you could merge this for 3.16? > --- > include/linux/kvm_host.h | 1 + > include/uapi/linux/kvm.h | 1 + > arch/x86/kvm/vmx.c | 4 ++++ > virt/kvm/eventfd.c | 16 ++++++++++++++++ > virt/kvm/kvm_main.c | 1 + > 5 files changed, 23 insertions(+) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index b8e9a43..c2eaa9f 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -163,6 +163,7 @@ enum kvm_bus { > KVM_MMIO_BUS, > KVM_PIO_BUS, > KVM_VIRTIO_CCW_NOTIFY_BUS, > + KVM_FAST_MMIO_BUS, > KVM_NR_BUSES > }; > > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h > index d73007e..fc75810 100644 > --- a/include/uapi/linux/kvm.h > +++ b/include/uapi/linux/kvm.h > @@ -450,6 +450,7 @@ enum { > kvm_ioeventfd_flag_nr_pio, > kvm_ioeventfd_flag_nr_deassign, > kvm_ioeventfd_flag_nr_virtio_ccw_notify, > + kvm_ioeventfd_flag_nr_fast_mmio, > kvm_ioeventfd_flag_nr_max, > }; > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > index 3927528..78a1932 100644 > --- a/arch/x86/kvm/vmx.c > +++ b/arch/x86/kvm/vmx.c > @@ -5493,6 +5493,10 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu) > gpa_t gpa; > > gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS); > + if (!kvm_io_bus_write(vcpu->kvm, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) { > + skip_emulated_instruction(vcpu); > + return 1; > + } > > ret = handle_mmio_page_fault_common(vcpu, gpa, true); > if (likely(ret == RET_MMIO_PF_EMULATE)) > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c > index 9eadb59..c61f2eb 100644 > --- a/virt/kvm/eventfd.c > +++ b/virt/kvm/eventfd.c > @@ -770,6 +770,16 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct > kvm_ioeventfd *args) > if (ret < 0) > goto unlock_fail; > > + /* When length is ignored, MMIO is also put on a separate bus, for > + * faster lookups. > + */ > + if (!args->len && !(args->flags & KVM_IOEVENTFD_FLAG_PIO)) { > + ret = kvm_io_bus_register_dev(kvm, KVM_FAST_MMIO_BUS, > + p->addr, 0, &p->dev); > + if (ret < 0) > + goto register_fail; > + } > + > kvm->buses[bus_idx]->ioeventfd_count++; > list_add_tail(&p->list, &kvm->ioeventfds); > > @@ -777,6 +787,8 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct > kvm_ioeventfd *args) > > return 0; > > +register_fail: > + kvm_io_bus_unregister_dev(kvm, bus_idx, &p->dev); > unlock_fail: > mutex_unlock(&kvm->slots_lock); > > @@ -816,6 +828,10 @@ kvm_deassign_ioeventfd(struct kvm *kvm, struct > kvm_ioeventfd *args) > continue; > > kvm_io_bus_unregister_dev(kvm, bus_idx, &p->dev); > + if (!p->length) { > + kvm_io_bus_unregister_dev(kvm, KVM_FAST_MMIO_BUS, > + &p->dev); > + } > kvm->buses[bus_idx]->ioeventfd_count--; > ioeventfd_release(p); > ret = 0; > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 03a0381..22a4e1b 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -2920,6 +2920,7 @@ static int __kvm_io_bus_read(struct kvm_io_bus *bus, > struct kvm_io_range *range, > > return -EOPNOTSUPP; > } > +EXPORT_SYMBOL_GPL(kvm_io_bus_write); > > /* kvm_io_bus_read - called under kvm->slots_lock */ > int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr, > -- > MST > > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html