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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Tue, 20 May 2014 12:25:36 +0200
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Subject: Re: [Qemu-devel] usb: usb tablet freeze when save/restore guest os
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To: "Gonglei (Arei)" <arei.gonglei@huawei.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, "xen-devel@lists.xen.org" <xen-devel@lists.xen.org>, "Huangweidong (C)" <weidong.huang@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Michael S. Tsirkin" <mst@redhat.com>

  Hi,

> As for Xen, It's too later. After suspend vm, the qemu process can response the event
> of usb tablet event. Because guest os's vcpus are paused, guest os cannot response 
> interrupt injected by qemu. Then the interrupt will be lost.

When the guest can't handle the interrupt (due to being stopped) it
should continue to be tagged pending in the ioapic.  And this should be
migrated over with the ioapic state, so the guest should receive and
handle the interrupt after being restarted to the target machine.

IIRC ioapic migration works differently in kvm and xen.  Maybe you've
hit a xen live migration bug.

cheers,
  Gerd