hotplug: VM got stuck when attaching a pass-through device to the non-pass-through VM for the first time

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Hi, all

The VM will get stuck for a while(about 6s for a VM with 20GB memory) when attaching a pass-through PCI card to the non-pass-through VM for the first time. 
The reason is that the host will build the whole VT-d GPA->HPA DMAR page-table, which needs a lot of time, and during this time, the qemu_global_mutex
lock is hold by the main-thread, if the vcpu thread IOCTL return, it will be blocked to waiting main-thread to release the qemu_global_mutex lock,
so the VM got stuck.
The race between qemu-main-thread and vcpu-thread is shown as below,

              QEMU-main-thread                                vcpu-thread           
                     |                                             |
      qemu_mutex_lock_iothread                     qemu_mutex_lock(&qemu_global_mutex)
                     |                                             |
        +----loop- ->+                               +----loop---->+               
        |            |                               |             |
        |  qemu_mutex_unlock_iothread                | qemu_mutex_unlock_iothread 
        |            |                               |             |              
        |           poll                             |    kvm_vcpu_ioctl(KVM_RUN) 
        |            |                               |             |              
        | qemu_mutex_lock_iothread                   |             |
        |            |                               |             | 
 --------------------------------------------------------------------------------------
        |            |                               |  qemu_mutex_lock_iothread
        |   kvm_device_pci_assign                    |             |              
        |            |                               |   blocked to waiting main-thread to release the qemu lock
        |      about 6 sec for 20GB memory           |             |              
        |            |                               |             |                     
        +------------+                               +-------------+              


Any advises?

Thanks,
Zhang Haoyu
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