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Date: Tue, 18 Feb 2014 12:51:59 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] hotplug: VM got stuck when attaching a
 pass-through device to the non-pass-through VM for the first time
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "chenliang (T)" <chenliang88@huawei.com>, Avi Kivity <avi@cloudius-systems.com>, "Huangweidong (C)" <weidong.huang@huawei.com>, Gleb Natapov <gleb@redhat.com>, KVM <kvm@vger.kernel.org>, "Zhanghaoyu (A)" <haoyu.zhang@huawei.com>, Luonengjun <luonengjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Gonglei (Arei)" <arei.gonglei@huawei.com>, "Gaowei (UVP)" <gao.gaowei@huawei.com>

On Tue, Feb 18, 2014 at 11:42:19AM +0100, Paolo Bonzini wrote:
> 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

Or the new shared flag - IIRC shared VMAs don't do COW either.