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 <mst@xxxxxxxxxx>
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
>
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