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Date: Wed, 5 Mar 2014 15:46:17 +0100
From: Andrea Arcangeli <aarcange@redhat.com>
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Subject: Re: [Qemu-devel] [RFC]VM live snapshot proposal
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To: "Huangpeng (Peter)" <peter.huangpeng@huawei.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Pavel Hrdina <phrdina@redhat.com>, Zhanghailiang <zhang.zhanghailiang@huawei.com>, KVM devel mailing list <kvm@vger.kernel.org>, Stefan Hajnoczi <stefanha@gmail.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>, Wenchao Xia <xiawenc@linux.vnet.ibm.com>

Hi,

On Tue, Mar 04, 2014 at 01:35:53AM +0000, Huangpeng (Peter) wrote:
>  
> > Hi Paolo,
> > 
> > On Mon, Mar 03, 2014 at 02:47:31PM +0100, Paolo Bonzini wrote:
> > >   I'm not sure what's the status of the kernel infrastructure for
> > > post-copy.  Andrea?
> > 
> > sys_userfaultfd is still work in progress but it shouldn't be much work left to
> > completion. madvise(MADV_USERFAULT) and
> > remap_anon_pages() are complete for a while.
> 
> http://qemu-project.org/Features/PostCopyLiveMigration
> From the feature description, post-copy uses memory copy, so this infrastructure
> will solve this problem, but do not help snapshot, am I right?

Correct there's no copy with this infrastructure, other than whatever
data copy that may be happening inside the network receive protocol
for skb linearization into userland memory. With RDMA or zerocopy DMA
receive mechanisms, there may be no copy at all.