From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: hotplug: VM got stuck when attaching a pass-through device to
 the non-pass-through VM for the first time
Date: Tue, 18 Feb 2014 11:42:19 +0100
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References: <D3E216785288A145B7BC975F83A2ED10448FC24D@SZXEMA510-MBS.china.huawei.com> <20140218103828.GB8114@redhat.com>
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Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	KVM <kvm@vger.kernel.org>, Gleb Natapov <gleb@redhat.com>,
	Avi Kivity <avi@cloudius-systems.com>,
	Eric Blake <eblake@redhat.com>,
	Luonengjun <luonengjun@huawei.com>,
	"Huangweidong (C)" <weidong.huang@huawei.com>,
	"Gonglei (Arei)" <arei.gonglei@huawei.com>,
	"Gaowei (UVP)" <gao.gaowei@huawei.com>,
	"chenliang (T)" <chenliang88@huawei.com>
To: "Michael S. Tsirkin" <mst@redhat.com>,
	"Zhanghaoyu (A)" <haoyu.zhang@huawei.com>
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Il 18/02/2014 11:38, Michael S. Tsirkin ha scritto:
> 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?

I agree it's either COW breaking or (similarly) locking pages that the 
guest hasn't touched yet.

You can use prealloc or "-rt mlock=on" to avoid this problem.

Paolo