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Re: [Qemu-devel] [PATCH v1 00/17] Background snapshots

From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH v1 00/17] Background snapshots
Date: Wed, 5 Sep 2018 11:32:55 +0800
User-agent: Mutt/1.10.1 (2018-07-13)

On Tue, Sep 04, 2018 at 04:00:31PM +0300, Denis Plotnikov wrote:
> Hi Peter

Hi, Denis,

> I moved the code to the repository
> https://github.com/denis-plotnikov/qemu/tree/background-snapshot-kvm.
> the current version includes fixes with respect to your comments for version
> 1.
> I moved KVM related patches to the end of the branch (formerly patch
> series).
> Since, the KVM patches and the other parts to modify (vhost an others) are
> needless in favor of upcoming userfaltfd,
> I would ask you to review the general framework which is able to work with
> tcg.
> Thanks in advance!

Thank you for pushing the tree.

I might have made a mistake before that I thought this work is at
least working for TCG, but I think I was wrong.  The problem is (I'm
trying to repeat Dave's question that you seems haven't yet answered):
even for TCG there could be use cases where the process might access
guest memory from the kernel space (e.g., vhost, or any system calls
that with a guest memory buffer passed in).  I'm afraid mprotect() and
the whole signal-based mechanism cannot be able to address these page
faults, then we'll encounter adhoc errors and we'll need to fix all
these places up.  Userfaultfd-wp should not have this problem.

I think the general idea of the work is good, but I'm not sure whether
we can merge the work if we don't settle these issues.


Peter Xu

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