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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