From 013cc6ebbf41496ce4badedd71ea6d4a6d198c14 Mon Sep 17 00:00:00 2001 From: Vitaly Kuznetsov Date: Wed, 13 Mar 2019 18:13:42 +0100 Subject: x86/kvm/hyper-v: avoid spurious pending stimer on vCPU init When userspace initializes guest vCPUs it may want to zero all supported MSRs including Hyper-V related ones including HV_X64_MSR_STIMERn_CONFIG/ HV_X64_MSR_STIMERn_COUNT. With commit f3b138c5d89a ("kvm/x86: Update SynIC timers on guest entry only") we began doing stimer_mark_pending() unconditionally on every config change. The issue I'm observing manifests itself as following: - Qemu writes 0 to STIMERn_{CONFIG,COUNT} MSRs and marks all stimers as pending in stimer_pending_bitmap, arms KVM_REQ_HV_STIMER; - kvm_hv_has_stimer_pending() starts returning true; - kvm_vcpu_has_events() starts returning true; - kvm_arch_vcpu_runnable() starts returning true; - when kvm_arch_vcpu_ioctl_run() gets into (vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED) case: - kvm_vcpu_block() gets in 'kvm_vcpu_check_block(vcpu) < 0' and returns immediately, avoiding normal wait path; - -EAGAIN is returned from kvm_arch_vcpu_ioctl_run() immediately forcing userspace to retry. So instead of normal wait path we get a busy loop on all secondary vCPUs before they get INIT signal. This seems to be undesirable, especially given that this happens even when Hyper-V extensions are not used. Generally, it seems to be pointless to mark an stimer as pending in stimer_pending_bitmap and arm KVM_REQ_HV_STIMER as the only thing kvm_hv_process_stimers() will do is clear the corresponding bit. We may just not mark disabled timers as pending instead. Fixes: f3b138c5d89a ("kvm/x86: Update SynIC timers on guest entry only") Signed-off-by: Vitaly Kuznetsov Signed-off-by: Paolo Bonzini --- arch/x86/kvm/hyperv.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) (limited to 'arch') diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c index 27c43525a05f..421899f6ad7b 100644 --- a/arch/x86/kvm/hyperv.c +++ b/arch/x86/kvm/hyperv.c @@ -526,7 +526,9 @@ static int stimer_set_config(struct kvm_vcpu_hv_stimer *stimer, u64 config, new_config.enable = 0; stimer->config.as_uint64 = new_config.as_uint64; - stimer_mark_pending(stimer, false); + if (stimer->config.enable) + stimer_mark_pending(stimer, false); + return 0; } @@ -542,7 +544,10 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count, stimer->config.enable = 0; else if (stimer->config.auto_enable) stimer->config.enable = 1; - stimer_mark_pending(stimer, false); + + if (stimer->config.enable) + stimer_mark_pending(stimer, false); + return 0; } -- cgit v1.2.3