[dpdk-dev] [PATCH] vhost: allow for many vhost user ports

[Jan Wickbom]

> -----Original Message-----
> From: Yuanhan Liu [mailto:yuanhan.liu at linux.intel.com]
> Sent: den 7 december 2016 11:13
> To: Jan Wickbom <jan.wickbom at ericsson.com>
> Cc: dev at dpdk.org; Patrik Andersson R <patrik.r.andersson at ericsson.com>
> Subject: Re: [PATCH] vhost: allow for many vhost user ports
> 
> On Thu, Dec 01, 2016 at 04:26:50PM +0100, Jan Wickbom wrote:
> >  static int
> > -fdset_fill(fd_set *rfset, fd_set *wfset, struct fdset *pfdset)
> > +fdset_fill(struct pollfd *rwfds, struct fdset *pfdset)
> >  {
> >  	struct fdentry *pfdentry;
> > -	int i, maxfds = -1;
> > -	int num = MAX_FDS;
> > -
> > -	if (pfdset == NULL)
> > -		return -1;
> > +	int i;
> > +	int num;
> >
> > -	for (i = 0; i < num; i++) {
> > +	for (i = 0, num = pfdset->num; i < num; i++) {
> >  		pfdentry = &pfdset->fd[i];
> > -		if (pfdentry->fd != -1) {
> > -			int added = 0;
> > -			if (pfdentry->rcb && rfset) {
> > -				FD_SET(pfdentry-
> >fd, rfset);
> > -				added = 1;
> > -			}
> > -			if (pfdentry->wcb && wfset) {
> > -				FD_SET(pfdentry-
> >fd, wfset);
> > -				added = 1;
> > -			}
> > -			if (added)
> > -				maxfds = pfdentry-
> >fd < maxfds ?
> > -
> 	maxfds : pfdentry->fd;
> > +
> > +		if (pfdentry->fd < 0) {
> > +			/* Hole in the list. Move the last
> one here */
> > +
> > +			*pfdentry = pfdset->fd[num - 1];
> > +			pfdset->fd[num - 1].fd = -1;
> > +			num =
> fdset_adjust_num(pfdset);
> >  		}
> > +		rwfds[i].fd = pfdentry->fd;
> > +		rwfds[i].events = pfdentry->rcb ? POLLIN : 0;
> > +		rwfds[i].events |= pfdentry->wcb ? POLLOUT :
> 0;
> 
> Another thing is we don't have to re-init this rwfds array again
> and again. Instead, we could
> 
> - set it up correctly when fdset_add is invoked: set the fd and
>   events.
> 
> - reset revents when it's been handled at fdset_event_dispatch().
> 
> - swap with the last one and shrink the array on fd delete
> 
> Could you make a follow up patch for that?

I don't see how that could easily be done. The loop index, i, is a direct reference between
an entry in the rwfds array and an entry in the pfdset array. It should stay like that while we are
hanging in the poll(). If  an entry in the pfdset array is removed while we are hanging in the poll()
and we then immediately replaces it with the last entry in the array we will end up in trouble if the
revent gets set for the "replaced" index. The direct reference is gone.
Or am I missing something?
/jaw
> 
> Thanks.
> 
> 	--yliu