Re: [Qemu-devel] [PATCH v4 07/22] vhost: alloc shareable log

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 21, 2015 at 11:44:38PM +0200, Marc-André Lureau wrote:
> Hi
> 
> On Mon, Sep 21, 2015 at 9:29 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >> Can this be considered a future enhancement?
> >
> > What's the big issue? Just count the devices that need a shared one, if
> > that count is 0 reallocate with shared == false.
> 
> 
> But then it should also VHOST_SET_LOG_BASE all the other devices with
> the new log, unless you want to tackle only the future log users. So
> it needs to track all the users of the log.

We already do this.
Same applies to non-memfd->memfd switch.

> Is there a clear benefit
> of this? since the memory isn't shared without the memfd passed to
> another process and the overhead of memfd is probably quite small, and
> pre-shm or future resize will not use the shared memory already.

For example, THP doesn't work for memfd at the moment,
so all accesses are a bit slower.

Really, I don't want to merge hacks. Switching from non memfd
to memfd but not back has all the signs of one.
Let's do it cleanly please.

> -- 
> Marc-André Lureau