Re: [PATCH] kvm: add capability for halt polling

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Jon Cargille <jcargill@google.com> writes:

> From: David Matlack <dmatlack@google.com>
>
> KVM_CAP_HALT_POLL is a per-VM capability that lets userspace
> control the halt-polling time, allowing halt-polling to be tuned or
> disabled on particular VMs.
>
> With dynamic halt-polling, a VM's VCPUs can poll from anywhere from
> [0, halt_poll_ns] on each halt. KVM_CAP_HALT_POLL sets the
> upper limit on the poll time.

Out of pure curiosity, why is this a per-VM and not a per-VCPU property?

>
> Signed-off-by: David Matlack <dmatlack@google.com>
> Signed-off-by: Jon Cargille <jcargill@google.com>
> Reviewed-by: Jim Mattson <jmattson@google.com>
> ---
>  Documentation/virt/kvm/api.rst | 17 +++++++++++++++++
>  include/linux/kvm_host.h       |  1 +
>  include/uapi/linux/kvm.h       |  1 +
>  virt/kvm/kvm_main.c            | 19 +++++++++++++++----
>  4 files changed, 34 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index efbbe570aa9b7b..d871dacb984e98 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -5802,6 +5802,23 @@ If present, this capability can be enabled for a VM, meaning that KVM
>  will allow the transition to secure guest mode.  Otherwise KVM will
>  veto the transition.
>  
> +7.20 KVM_CAP_HALT_POLL
> +----------------------
> +
> +:Architectures: all
> +:Target: VM
> +:Parameters: args[0] is the maximum poll time in nanoseconds
> +:Returns: 0 on success; -1 on error
> +
> +This capability overrides the kvm module parameter halt_poll_ns for the
> +target VM.
> +
> +VCPU polling allows a VCPU to poll for wakeup events instead of immediately
> +scheduling during guest halts. The maximum time a VCPU can spend polling is
> +controlled by the kvm module parameter halt_poll_ns. This capability allows
> +the maximum halt time to specified on a per-VM basis, effectively overriding
> +the module parameter for the target VM.
> +
>  8. Other capabilities.
>  ======================
>  
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 6d58beb65454f7..922b24ce5e7297 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -503,6 +503,7 @@ struct kvm {
>  	struct srcu_struct srcu;
>  	struct srcu_struct irq_srcu;
>  	pid_t userspace_pid;
> +	unsigned int max_halt_poll_ns;
>  };
>  
>  #define kvm_err(fmt, ...) \
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 428c7dde6b4b37..ac9eba0289d1b6 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1017,6 +1017,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_S390_VCPU_RESETS 179
>  #define KVM_CAP_S390_PROTECTED 180
>  #define KVM_CAP_PPC_SECURE_GUEST 181
> +#define KVM_CAP_HALT_POLL 182
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 74bdb7bf32952e..ec038a9e60a275 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -710,6 +710,8 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  			goto out_err_no_arch_destroy_vm;
>  	}
>  
> +	kvm->max_halt_poll_ns = halt_poll_ns;
> +
>  	r = kvm_arch_init_vm(kvm, type);
>  	if (r)
>  		goto out_err_no_arch_destroy_vm;
> @@ -2716,15 +2718,16 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  	if (!kvm_arch_no_poll(vcpu)) {
>  		if (!vcpu_valid_wakeup(vcpu)) {
>  			shrink_halt_poll_ns(vcpu);
> -		} else if (halt_poll_ns) {
> +		} else if (vcpu->kvm->max_halt_poll_ns) {
>  			if (block_ns <= vcpu->halt_poll_ns)
>  				;
>  			/* we had a long block, shrink polling */
> -			else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
> +			else if (vcpu->halt_poll_ns &&
> +					block_ns > vcpu->kvm->max_halt_poll_ns)
>  				shrink_halt_poll_ns(vcpu);
>  			/* we had a short halt and our poll time is too small */
> -			else if (vcpu->halt_poll_ns < halt_poll_ns &&
> -				block_ns < halt_poll_ns)
> +			else if (vcpu->halt_poll_ns < vcpu->kvm->max_halt_poll_ns &&
> +					block_ns < vcpu->kvm->max_halt_poll_ns)
>  				grow_halt_poll_ns(vcpu);
>  		} else {
>  			vcpu->halt_poll_ns = 0;
> @@ -3516,6 +3519,7 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
>  	case KVM_CAP_IOEVENTFD_ANY_LENGTH:
>  	case KVM_CAP_CHECK_EXTENSION_VM:
>  	case KVM_CAP_ENABLE_CAP_VM:
> +	case KVM_CAP_HALT_POLL:
>  		return 1;
>  #ifdef CONFIG_KVM_MMIO
>  	case KVM_CAP_COALESCED_MMIO:
> @@ -3566,6 +3570,13 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
>  		return 0;
>  	}
>  #endif
> +	case KVM_CAP_HALT_POLL: {
> +		if (cap->flags || cap->args[0] != (unsigned int)cap->args[0])
> +			return -EINVAL;
> +
> +		kvm->max_halt_poll_ns = cap->args[0];

Is it safe to allow any value from userspace here or would it maybe make
sense to only allow [0, global halt_poll_ns]?


> +		return 0;
> +	}
>  	default:
>  		return kvm_vm_ioctl_enable_cap(kvm, cap);
>  	}

-- 
Vitaly