Re: [Qemu-devel] [PATCH v4 for-4.0 0/7] vhost-user-blk: Add support for backend reconnecting

[Yongji Xie]

On Thu, 10 Jan 2019 at 18:25, Stefan Hajnoczi <address@hidden> wrote:
>
> On Wed, Jan 09, 2019 at 07:27:21PM +0800, address@hidden wrote:
> > From: Xie Yongji <address@hidden>
> >
> > This patchset is aimed at supporting qemu to reconnect
> > vhost-user-blk backend after vhost-user-blk backend crash or
> > restart.
> >
> > The patch 1 uses exisiting wait/nowait options to make QEMU not
> > do a connect on client sockets during initialization of the chardev.
> >
> > The patch 2 introduces two new messages VHOST_USER_GET_INFLIGHT_FD
> > and VHOST_USER_SET_INFLIGHT_FD to support providing shared
> > memory to backend.
>
> Can you describe the problem that the inflight I/O shared memory region
> solves?
>

The backend need to get inflight I/O and do I/O replay after restart.
Now we can only get used_idx in used_ring. It's not enough. Because we
might not process descriptors in the same order which they have been
made available sometimes. A simple example:
https://patchwork.kernel.org/cover/10715305/#22375607. So we need a
shared memory to track inflight I/O.

Thanks,
Yongji