2018-02-01 16:48+0300, Roman Kagan:
> In Hyper-V, the fast guest->host notification mechanism is the
> SIGNAL_EVENT hypercall, with a single parameter of the connection ID to
> signal.
>
> Currently this hypercall incurs a user exit and requires the userspace
> to decode the parameters and trigger the notification of the potentially
> different I/O context.
>
> To avoid the costly user exit, process this hypercall and signal the
> corresponding eventfd in KVM, similar to ioeventfd. The association
> between the connection id and the eventfd is established via the newly
> introduced KVM_HYPERV_EVENTFD ioctl, and maintained in an
> (srcu-protected) IDR.
>
> Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
Queued, thanks.
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index fc3ae951bc07..3a3fbcc3377d 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -3449,6 +3449,37 @@ array bounds check and the array access.
> These fields use the same bit definitions as the new
> H_GET_CPU_CHARACTERISTICS hypercall.
>
> +4.110 KVM_HYPERV_EVENTFD
> +
> +Capability: KVM_CAP_HYPERV_EVENTFD
> +Architectures: x86
> +Type: vm ioctl
> +Parameters: struct kvm_hyperv_eventfd (in)
> +
> +This ioctl (un)registers an eventfd to receive notifications from the guest on
> +the specified Hyper-V connection id through the SIGNAL_EVENT hypercall, without
> +causing a user exit.
I've added:
+ SIGNAL_EVENT hypercall with non-zero event flag number
+ (bits 24-31) still triggers a KVM_EXIT_HYPERV_HCALL VM exit.
We might have to extend the interface to optimize this case in the
future, but at least it's not going to bug.
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> @@ -934,6 +934,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_S390_AIS_MIGRATION 150
> #define KVM_CAP_PPC_GET_CPU_CHAR 151
> #define KVM_CAP_S390_BPB 152
> +#define KVM_CAP_HYPERV_EVENTFD 153
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> @@ -1363,6 +1364,8 @@ struct kvm_s390_ucas_mapping {
> #define KVM_S390_GET_CMMA_BITS _IOWR(KVMIO, 0xb8, struct kvm_s390_cmma_log)
> #define KVM_S390_SET_CMMA_BITS _IOW(KVMIO, 0xb9, struct kvm_s390_cmma_log)
>
> +#define KVM_HYPERV_EVENTFD _IOW(KVMIO, 0xba, struct kvm_hyperv_eventfd)
The hypercall number is 0xbd for now.
(Might change until the patches get to kvm/next and the capability
number is also prone to changing).
