KVM: Obey kvm.halt_poll_ns in VMs not using KVM_CAP_HALT_POLL

Obey kvm.halt_poll_ns in VMs not using KVM_CAP_HALT_POLL on every halt,
rather than just sampling the module parameter when the VM is first
created. This restore the original behavior of kvm.halt_poll_ns for VMs
that have not opted into KVM_CAP_HALT_POLL.

Notably, this change restores the ability for admins to disable or
change the maximum halt-polling time system wide for VMs not using
KVM_CAP_HALT_POLL.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Fixes: acd05785e48c ("kvm: add capability for halt polling")
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20221117001657.1067231-4-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 18592bd..637a606 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -776,6 +776,7 @@ struct kvm {
        struct srcu_struct srcu;
        struct srcu_struct irq_srcu;
        pid_t userspace_pid;
+       bool override_halt_poll_ns;
        unsigned int max_halt_poll_ns;
        u32 dirty_ring_size;
        bool vm_bugged;

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 032d2fa..fab4d37 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1198,8 +1198,6 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
                        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;
@@ -3482,7 +3480,20 @@ static inline void update_halt_poll_stats(struct kvm_vcpu *vcpu, ktime_t start,
 
 static unsigned int kvm_vcpu_max_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-       return READ_ONCE(vcpu->kvm->max_halt_poll_ns);
+       struct kvm *kvm = vcpu->kvm;
+
+       if (kvm->override_halt_poll_ns) {
+               /*
+                * Ensure kvm->max_halt_poll_ns is not read before
+                * kvm->override_halt_poll_ns.
+                *
+                * Pairs with the smp_wmb() when enabling KVM_CAP_HALT_POLL.
+                */
+               smp_rmb();
+               return READ_ONCE(kvm->max_halt_poll_ns);
+       }
+
+       return READ_ONCE(halt_poll_ns);
 }
 
 /*
@@ -4592,6 +4603,16 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
                        return -EINVAL;
 
                kvm->max_halt_poll_ns = cap->args[0];
+
+               /*
+                * Ensure kvm->override_halt_poll_ns does not become visible
+                * before kvm->max_halt_poll_ns.
+                *
+                * Pairs with the smp_rmb() in kvm_vcpu_max_halt_poll_ns().
+                */
+               smp_wmb();
+               kvm->override_halt_poll_ns = true;
+
                return 0;
        }
        case KVM_CAP_DIRTY_LOG_RING: