Re: [Qemu-devel] hotplug: VM got stuck when attaching a pass-through device to the non-pass-through VM for the first time

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 18, 2014 at 02:38:40AM +0000, Zhanghaoyu (A) wrote:
> 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

What if you detach and re-attach?
Is it fast then?
If yes this means the issue is COW breaking that occurs
with get_user_pages, not translation as such.
Try hugepages with prealloc - does it help?




-- 
MST