From 68c3b4d1676d870f0453c31d5a52e7e65c7448ae Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Mon, 31 Mar 2014 21:50:44 +0300 Subject: [PATCH] KVM: VMX: speed up wildcard MMIO EVENTFD 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 Signed-off-by: Marcelo Tosatti --- arch/x86/kvm/vmx.c | 4 ++++ include/linux/kvm_host.h | 1 + include/uapi/linux/kvm.h | 1 + virt/kvm/eventfd.c | 16 ++++++++++++++++ virt/kvm/kvm_main.c | 1 + 5 files changed, 23 insertions(+) diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index 1f68c5831924..eb3f2b1b764c 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -5528,6 +5528,10 @@ 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/include/linux/kvm_host.h b/include/linux/kvm_host.h index 7d21cf9f4380..6c3c2eb96d06 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 39098a61f41c..d8a6ce4c2a83 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -515,6 +515,7 @@ 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/virt/kvm/eventfd.c b/virt/kvm/eventfd.c index 2721996bb9c2..912ec5a95e2c 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 56baae8c2f56..96456ac888ba 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -2922,6 +2922,7 @@ 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, -- 2.20.1