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

[Anton Nefedov]

On 07/05/2017 06:09 PM, Paolo Bonzini wrote:
> 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

Hi Paolo,

So instead we'll need to use proper locks in each of the front-ends?
Or do you mean that it can be skipped for the most of them? I don't know
about all possible threading cases.
e.g. for serial/virtio-serial? Will they always share the same thread
with hmp/qmp driven chardev-change command? And won't yield and hotswap
in the middle of some write handler?

/Anton