kvm: add capability for halt polling

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.

Signed-off-by: David Matlack <dmatlack@google.com>
Signed-off-by: Jon Cargille <jcargill@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Message-Id: <20200417221446.108733-1-jcargill@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index efbbe57..d871dac 100644 (file)
--- 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 5285a55..3cc6ccb 100644 (file)
--- 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 428c7dd..ac9eba0 100644 (file)
--- 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 e7436d0..33e1eee 100644 (file)
--- 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;
@@ -2713,15 +2715,16 @@ out:
        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;
@@ -3510,6 +3513,7 @@ 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:
@@ -3560,6 +3564,13 @@ 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];
+               return 0;
+       }
        default:
                return kvm_vm_ioctl_enable_cap(kvm, cap);
        }