netdev - Re: [PATCH 2/2] vhost-net: revert vhost_exceeds

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Message-ID: <1503322205.8694.7.camel@klaipeden.com>
Date:   Mon, 21 Aug 2017 22:30:05 +0900
From:   Koichiro Den <den@...ipeden.com>
To:     Jason Wang <jasowang@...hat.com>, mst@...hat.com
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] vhost-net: revert vhost_exceeds_maxpend logic to
 its original

On Mon, 2017-08-21 at 11:06 +0800, Jason Wang wrote:
> 
> On 2017年08月19日 14:41, Koichiro Den wrote:
> > To depend on vq.num and the usage of VHOST_MAX_PEND is not succinct
> > and in some case unexpected, so revert its logic part only.
> 
> Hi:
> 
> Could you explain a little bit more on the case that is was not sufficent?
It's just because vq.num could theoretically be around VHOST_MAX_PEND, that
could lead to an unexpected situation. Plus, I thought its name and usage is not
intrinsic. Its actual functioning in normal vq.num == 256 case is I think good
enough. Thanks.
> 
> Thanks
> 
> > 
> > Signed-off-by: Koichiro Den <den@...ipeden.com>
> > ---
> >   drivers/vhost/net.c | 8 ++++++--
> >   1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 06d044862e58..99cf99b308a7 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -433,11 +433,15 @@ static int vhost_net_tx_get_vq_desc(struct vhost_net
> > *net,
> >   
> >   static bool vhost_exceeds_maxpend(struct vhost_net *net)
> >   {
> > +	int num_pends;
> >   	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
> >   	struct vhost_virtqueue *vq = &nvq->vq;
> >   
> > -	return (nvq->upend_idx + vq->num - VHOST_MAX_PEND) % UIO_MAXIOV
> > -		== nvq->done_idx;
> > +	num_pends = likely(nvq->upend_idx >= nvq->done_idx) ?
> > +		(nvq->upend_idx - nvq->done_idx) :
> > +		(nvq->upend_idx + UIO_MAXIOV - nvq->done_idx);
> > +
> > +	return num_pends > VHOST_MAX_PEND;
> >   }
> >   
> >   /* Expects to be always run from workqueue - which acts as
> 
>