Re: [PATCH V3 00/22] Live Update

[Stefan Hajnoczi]

On Fri, May 07, 2021 at 05:24:58AM -0700, Steve Sistare wrote:
> Provide the cprsave and cprload commands for live update.  These save and
> restore VM state, with minimal guest pause time, so that qemu may be updated
> to a new version in between.
> 
> cprsave stops the VM and saves vmstate to an ordinary file.  It supports two
> modes: restart and reboot.  For restart, cprsave exec's the qemu binary (or
> /usr/bin/qemu-exec if it exists) with the same argv.  qemu restarts in a
> paused state and waits for the cprload command.

I think cprsave/cprload could be generalized by using QMP to stash the
file descriptors. The 'getfd' QMP command already exists and QEMU code
already opens fds passed using this mechanism.

I haven't checked but it may be possible to drop some patches by reusing
QEMU's monitor file descriptor passing since the code already knows how
to open from 'getfd' fds.

The reason why using QMP is interesting is because it eliminates the
need for execve(2). QEMU may be unable to execute a program due to
chroot, seccomp, etc.

QMP would enable cprsave/cprload to work both with and without
execve(2).

One tricky thing with this approach might be startup ordering: how to
get fds via the QMP monitor in the new process before processing the
entire command-line.

Stefan

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