KVM: race-free exit from KVM_RUN without POSIX signals
The purpose of the KVM_SET_SIGNAL_MASK API is to let userspace "kick" a VCPU out of KVM_RUN through a POSIX signal. A signal is attached to a dummy signal handler; by blocking the signal outside KVM_RUN and unblocking it inside, this possible race is closed: VCPU thread service thread -------------------------------------------------------------- check flag set flag raise signal (signal handler does nothing) KVM_RUN However, one issue with KVM_SET_SIGNAL_MASK is that it has to take tsk->sighand->siglock on every KVM_RUN. This lock is often on a remote NUMA node, because it is on the node of a thread's creator. Taking this lock can be very expensive if there are many userspace exits (as is the case for SMP Windows VMs without Hyper-V reference time counter). As an alternative, we can put the flag directly in kvm_run so that KVM can see it: VCPU thread service thread -------------------------------------------------------------- raise signal signal handler set run->immediate_exit KVM_RUN check run->immediate_exit Reviewed-by: Radim Krčmář <rkrcmar@redhat.com> Reviewed-by: David Hildenbrand <david@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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@ -3389,7 +3389,18 @@ struct kvm_run {
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Request that KVM_RUN return when it becomes possible to inject external
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interrupts into the guest. Useful in conjunction with KVM_INTERRUPT.
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__u8 padding1[7];
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__u8 immediate_exit;
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This field is polled once when KVM_RUN starts; if non-zero, KVM_RUN
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exits immediately, returning -EINTR. In the common scenario where a
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signal is used to "kick" a VCPU out of KVM_RUN, this field can be used
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to avoid usage of KVM_SET_SIGNAL_MASK, which has worse scalability.
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Rather than blocking the signal outside KVM_RUN, userspace can set up
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a signal handler that sets run->immediate_exit to a non-zero value.
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This field is ignored if KVM_CAP_IMMEDIATE_EXIT is not available.
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__u8 padding1[6];
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/* out */
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__u32 exit_reason;
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@ -206,6 +206,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
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case KVM_CAP_ARM_PSCI_0_2:
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case KVM_CAP_READONLY_MEM:
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case KVM_CAP_MP_STATE:
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case KVM_CAP_IMMEDIATE_EXIT:
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r = 1;
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break;
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case KVM_CAP_COALESCED_MMIO:
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@ -604,6 +605,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
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return ret;
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}
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if (run->immediate_exit)
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return -EINTR;
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if (vcpu->sigset_active)
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sigprocmask(SIG_SETMASK, &vcpu->sigset, &sigsaved);
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@ -397,7 +397,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
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int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
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{
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int r = 0;
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int r = -EINTR;
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sigset_t sigsaved;
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if (vcpu->sigset_active)
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@ -409,6 +409,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
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vcpu->mmio_needed = 0;
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}
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if (run->immediate_exit)
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goto out;
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lose_fpu(1);
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local_irq_disable();
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@ -429,6 +432,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
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guest_exit_irqoff();
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local_irq_enable();
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out:
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if (vcpu->sigset_active)
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sigprocmask(SIG_SETMASK, &sigsaved, NULL);
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@ -1021,6 +1025,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
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case KVM_CAP_ENABLE_CAP:
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case KVM_CAP_READONLY_MEM:
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case KVM_CAP_SYNC_MMU:
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case KVM_CAP_IMMEDIATE_EXIT:
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r = 1;
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break;
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case KVM_CAP_COALESCED_MMIO:
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@ -511,6 +511,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
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case KVM_CAP_ONE_REG:
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case KVM_CAP_IOEVENTFD:
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case KVM_CAP_DEVICE_CTRL:
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case KVM_CAP_IMMEDIATE_EXIT:
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r = 1;
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break;
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case KVM_CAP_PPC_PAIRED_SINGLES:
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@ -1117,7 +1118,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
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#endif
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}
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r = kvmppc_vcpu_run(run, vcpu);
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if (run->immediate_exit)
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r = -EINTR;
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else
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r = kvmppc_vcpu_run(run, vcpu);
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if (vcpu->sigset_active)
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sigprocmask(SIG_SETMASK, &sigsaved, NULL);
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@ -370,6 +370,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
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case KVM_CAP_S390_IRQCHIP:
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case KVM_CAP_VM_ATTRIBUTES:
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case KVM_CAP_MP_STATE:
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case KVM_CAP_IMMEDIATE_EXIT:
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case KVM_CAP_S390_INJECT_IRQ:
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case KVM_CAP_S390_USER_SIGP:
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case KVM_CAP_S390_USER_STSI:
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@ -2798,6 +2799,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
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int rc;
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sigset_t sigsaved;
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if (kvm_run->immediate_exit)
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return -EINTR;
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if (guestdbg_exit_pending(vcpu)) {
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kvm_s390_prepare_debug_exit(vcpu);
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return 0;
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@ -2672,6 +2672,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
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case KVM_CAP_DISABLE_QUIRKS:
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case KVM_CAP_SET_BOOT_CPU_ID:
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case KVM_CAP_SPLIT_IRQCHIP:
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case KVM_CAP_IMMEDIATE_EXIT:
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#ifdef CONFIG_KVM_DEVICE_ASSIGNMENT
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case KVM_CAP_ASSIGN_DEV_IRQ:
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case KVM_CAP_PCI_2_3:
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@ -7202,7 +7203,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
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} else
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WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
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r = vcpu_run(vcpu);
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if (kvm_run->immediate_exit)
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r = -EINTR;
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else
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r = vcpu_run(vcpu);
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out:
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post_kvm_run_save(vcpu);
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@ -218,7 +218,8 @@ struct kvm_hyperv_exit {
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struct kvm_run {
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/* in */
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__u8 request_interrupt_window;
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__u8 padding1[7];
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__u8 immediate_exit;
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__u8 padding1[6];
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/* out */
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__u32 exit_reason;
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@ -881,6 +882,7 @@ struct kvm_ppc_resize_hpt {
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#define KVM_CAP_SPAPR_RESIZE_HPT 133
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#define KVM_CAP_PPC_MMU_RADIX 134
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#define KVM_CAP_PPC_MMU_HASH_V3 135
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#define KVM_CAP_IMMEDIATE_EXIT 136
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#ifdef KVM_CAP_IRQ_ROUTING
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