• Radim Krčmář's avatar
    KVM: x86: detect SPIV changes under APICv · e462755c
    Radim Krčmář authored
    APIC-write VM exits are "trap-like": they save CS:RIP values for the
    instruction after the write, and more importantly, the handler will
    already see the new value in the virtual-APIC page.
    
    This caused a bug if you used KVM_SET_IRQCHIP to set the SW-enabled bit
    in the SPIV register.  The chain of events is as follows:
    
    * When the irqchip is added to the destination VM, the apic_sw_disabled
    static key is incremented (1)
    
    * When the KVM_SET_IRQCHIP ioctl is invoked, it is decremented (0)
    
    * When the guest disables the bit in the SPIV register, e.g. as part of
    shutdown, apic_set_spiv does not notice the change and the static key is
    _not_ incremented.
    
    * When the guest is destroyed, the static key is decremented (-1),
    resulting in this trace:
    
      WARNING: at kernel/jump_label.c:81 __static_key_slow_dec+0xa6/0xb0()
      jump label: negative count!
    
      [<ffffffff816bf898>] dump_stack+0x19/0x1b
      [<ffffffff8107c6f1>] warn_slowpath_common+0x61/0x80
      [<ffffffff8107c76c>] warn_slowpath_fmt+0x5c/0x80
      [<ffffffff811931e6>] __static_key_slow_dec+0xa6/0xb0
      [<ffffffff81193226>] static_key_slow_dec_deferred+0x16/0x20
      [<ffffffffa0637698>] kvm_free_lapic+0x88/0xa0 [kvm]
      [<ffffffffa061c63e>] kvm_arch_vcpu_uninit+0x2e/0xe0 [kvm]
      [<ffffffffa05ff301>] kvm_vcpu_uninit+0x21/0x40 [kvm]
      [<ffffffffa067cec7>] vmx_free_vcpu+0x47/0x70 [kvm_intel]
      [<ffffffffa061bc50>] kvm_arch_vcpu_free+0x50/0x60 [kvm]
      [<ffffffffa061ca22>] kvm_arch_destroy_vm+0x102/0x260 [kvm]
      [<ffffffff810b68fd>] ? synchronize_srcu+0x1d/0x20
      [<ffffffffa06030d1>] kvm_put_kvm+0xe1/0x1c0 [kvm]
      [<ffffffffa06036f8>] kvm_vcpu_release+0x18/0x20 [kvm]
      [<ffffffff81215c62>] __fput+0x102/0x310
      [<ffffffff81215f4e>] ____fput+0xe/0x10
      [<ffffffff810ab664>] task_work_run+0xb4/0xe0
      [<ffffffff81083944>] do_exit+0x304/0xc60
      [<ffffffff816c8dfc>] ? _raw_spin_unlock_irq+0x2c/0x50
      [<ffffffff810fd22d>] ?  trace_hardirqs_on_caller+0xfd/0x1c0
      [<ffffffff8108432c>] do_group_exit+0x4c/0xc0
      [<ffffffff810843b4>] SyS_exit_group+0x14/0x20
      [<ffffffff816d33a9>] system_call_fastpath+0x16/0x1b
    Signed-off-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    e462755c
lapic.h 5.18 KB