Re: [Qemu-devel] [PATCH v5 03/13] char: chardevice hotswap

[Paolo Bonzini]

On 05/07/2017 16:01, Anton Nefedov wrote:
> This patch adds a possibility to change a char device without a frontend
> removal.
> 
> 1. Ideally, it would have to happen transparently to a frontend, i.e.
> frontend would continue its regular operation.
> However, backends are not stateless and are set up by the frontends
> via qemu_chr_fe_<> functions, and it's not (generally) possible to replay
> that setup entirely in a backend code, as different chardevs respond
> to the setup calls differently, so do frontends work differently basing
> on those setup responses.
> Moreover, some frontend can generally get and save the backend pointer
> (qemu_chr_fe_get_driver()), and it will become invalid after backend change.
> 
> So, a frontend which would like to support chardev hotswap has to register
> a "backend change" handler, and redo its backend setup there.
> 
> 2. Write path can be used by multiple threads and thus protected with
> chr_write_lock.
> So hotswap also has to be protected so write functions won't access
> a backend being replaced.

Does this matter in practice?  CharBackend thread safety can be left to
the front-end.

Paolo