Re: [Qemu-devel] [PATCH] vhost-user spec: Clarify policy on setting log_base

[Jason Wang]

On 2017年11月29日 02:06, Michael S. Tsirkin wrote:
> On Tue, Nov 28, 2017 at 03:46:44PM +0200, Victor Kaplansky wrote:
> > From: Victor Kaplansky<address@hidden>
> >
> > If we allow qemu to change logging area after it was already established,
> > it may require from the backend to acquire a lock on each access to
> > the log_base, which has a potential quite a big performance hit.
> >
> > Thus we would like to clarify in the spec, that qemu is not expected
> > to resize or remap the logging area, and backend implementations
> > can safely ignore subsequent requests to log_base modifications.
> >
> > Signed-off-by: Victor Kaplansky<address@hidden>
> > Suggested-by: Maxime Coquelin<address@hidden>
> I'm not sure we can do this.
>
>
> Log resizing as a result of memory hotplug might force
> log base changes.
>
> Backends need to use something like
> rcu to avoid need for locking.
>
> Apropos I wonder whether it's a bug that vhost_dev_start
> calls vhost_set_log_base after starting rings.
>
> Same question for the iotlb callback.

At least for kernel vhost-net, it doesn't since the backend won't start 
to work until SET_BACKEND which is done in vhost_net_start_one() after 
vhost_dev_start().

Thanks