vmm(4): allocate reference for vm and vcpu SLISTs

Mischa Peters reported a performance regression in 7.2 when hosting
numerous guests under vmm(4). While iterating through the list of
vms during servicing an ioctl, vmm was triggering excessive wakeup
calls due to hitting zero refcnt.

Much guidance from dlg@ and testing from Mischa. OK mlarkin@.

diff --git a/sys/arch/amd64/amd64/vmm.c b/sys/arch/amd64/amd64/vmm.c
index ee06cb0..d17038f 100644 (file)
--- a/sys/arch/amd64/amd64/vmm.c
+++ b/sys/arch/amd64/amd64/vmm.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: vmm.c,v 1.324 2022/11/01 01:01:14 cheloha Exp $       */
+/*     $OpenBSD: vmm.c,v 1.325 2022/11/06 19:00:37 dv Exp $    */
 /*
  * Copyright (c) 2014 Mike Larkin <mlarkin@openbsd.org>
  *
@@ -840,11 +840,9 @@ vm_resetcpu(struct vm_resetcpu_params *vrp)
        }
 
        rw_enter_write(&vcpu->vc_lock);
-
-       if (vcpu->vc_state != VCPU_STATE_STOPPED ||
-           refcnt_shared(&vcpu->vc_refcnt)) {
+       if (vcpu->vc_state != VCPU_STATE_STOPPED)
                ret = EBUSY;
-       } else {
+       else {
                if (vcpu_reset_regs(vcpu, &vrp->vrp_init_state)) {
                        printf("%s: failed\n", __func__);
 #ifdef VMM_DEBUG
@@ -1784,7 +1782,6 @@ vm_create(struct vm_create_params *vcp, struct proc *p)
        for (i = 0; i < vcp->vcp_ncpus; i++) {
                vcpu = pool_get(&vcpu_pool, PR_WAITOK | PR_ZERO);
                refcnt_init(&vcpu->vc_refcnt);
-               refcnt_rele(&vcpu->vc_refcnt);
 
                vcpu->vc_parent = vm;
                if ((ret = vcpu_init(vcpu)) != 0) {
@@ -1794,6 +1791,7 @@ vm_create(struct vm_create_params *vcp, struct proc *p)
                }
                vcpu->vc_id = vm->vm_vcpu_ct;
                vm->vm_vcpu_ct++;
+               /* Publish vcpu to list, inheriting the reference. */
                SLIST_INSERT_HEAD(&vm->vm_vcpu_list, vcpu, vc_vcpu_link);
        }
 
@@ -1816,12 +1814,13 @@ vm_create(struct vm_create_params *vcp, struct proc *p)
        vm->vm_id = vmm_softc->vm_idx;
        vcp->vcp_id = vm->vm_id;
 
-       /* Publish the vm into the list and update list count. */
-       SLIST_INSERT_HEAD(&vmm_softc->vm_list, vm, vm_link);
+       /* Update list counts. */
        vmm_softc->vm_ct++;
        vmm_softc->vcpu_ct += vm->vm_vcpu_ct;
 
-       refcnt_rele(&vm->vm_refcnt);            /* No need for wake. */
+       /* Publish vm into list, inheriting the reference. */
+       SLIST_INSERT_HEAD(&vmm_softc->vm_list, vm, vm_link);
+
        rw_exit_write(&vmm_softc->vm_lock);
 
        return (0);
@@ -4067,9 +4066,7 @@ vm_teardown(struct vm **target)
        /* Free VCPUs */
        rw_enter_write(&vm->vm_vcpu_lock);
        SLIST_FOREACH_SAFE(vcpu, &vm->vm_vcpu_list, vc_vcpu_link, tmp) {
-               refcnt_take(&vcpu->vc_refcnt);
                refcnt_finalize(&vcpu->vc_refcnt, "vcputeardown");
-
                SLIST_REMOVE(&vm->vm_vcpu_list, vcpu, vcpu, vc_vcpu_link);
                vcpu_deinit(vcpu);
 
@@ -4479,6 +4476,7 @@ vm_terminate(struct vm_terminate_params *vtp)
        rw_enter_write(&vmm_softc->vm_lock);
        SLIST_REMOVE(&vmm_softc->vm_list, vm, vm, vm_link);
        rw_exit_write(&vmm_softc->vm_lock);
+       refcnt_rele_wake(&vm->vm_refcnt);       /* Drop list reference. */
 
        vm_id = vm->vm_id;
        nvcpu = vm->vm_vcpu_ct;