Subject: Re: [PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 14 Dec 2018 07:36:45 -0500

On Fri, Dec 14, 2018 at 11:57:35AM +0800, Jason Wang wrote:
> 
> On 2018/12/13 下午11:44, Michael S. Tsirkin wrote:
> > On Thu, Dec 13, 2018 at 06:10:22PM +0800, Jason Wang wrote:
> > > It was noticed that the copy_user() friends that was used to access
> > > virtqueue metdata tends to be very expensive for dataplane
> > > implementation like vhost since it involves lots of software check,
> > > speculation barrier, hardware feature toggling (e.g SMAP). The
> > > extra cost will be more obvious when transferring small packets.
> > > 
> > > This patch tries to eliminate those overhead by pin vq metadata pages
> > > and access them through vmap(). During SET_VRING_ADDR, we will setup
> > > those mappings and memory accessors are modified to use pointers to
> > > access the metadata directly.
> > > 
> > > Note, this was only done when device IOTLB is not enabled. We could
> > > use similar method to optimize it in the future.
> > > 
> > > Tests shows about ~24% improvement on TX PPS when using virtio-user +
> > > vhost_net + xdp1 on TAP (CONFIG_HARDENED_USERCOPY is not enabled):
> > > 
> > > Before: ~5.0Mpps
> > > After:  ~6.1Mpps
> > > 
> > > Signed-off-by: Jason Wang<[email-masked]>
> > > ---
> > >   drivers/vhost/vhost.c | 178 ++++++++++++++++++++++++++++++++++++++++++
> > >   drivers/vhost/vhost.h |  11 +++
> > >   2 files changed, 189 insertions(+)
> > > 
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index bafe39d2e637..1bd24203afb6 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -443,6 +443,9 @@ void vhost_dev_init(struct vhost_dev *dev,
> > >   		vq->indirect = NULL;
> > >   		vq->heads = NULL;
> > >   		vq->dev = dev;
> > > +		memset(&vq->avail_ring, 0, sizeof(vq->avail_ring));
> > > +		memset(&vq->used_ring, 0, sizeof(vq->used_ring));
> > > +		memset(&vq->desc_ring, 0, sizeof(vq->desc_ring));
> > >   		mutex_init(&vq->mutex);
> > >   		vhost_vq_reset(dev, vq);
> > >   		if (vq->handle_kick)
> > > @@ -614,6 +617,102 @@ static void vhost_clear_msg(struct vhost_dev *dev)
> > >   	spin_unlock(&dev->iotlb_lock);
> > >   }
> > > +static int vhost_init_vmap(struct vhost_vmap *map, unsigned long uaddr,
> > > +			   size_t size, int write)
> > > +{
> > > +	struct page **pages;
> > > +	int npages = DIV_ROUND_UP(size, PAGE_SIZE);
> > > +	int npinned;
> > > +	void *vaddr;
> > > +
> > > +	pages = kmalloc_array(npages, sizeof(struct page *), GFP_KERNEL);
> > > +	if (!pages)
> > > +		return -ENOMEM;
> > > +
> > > +	npinned = get_user_pages_fast(uaddr, npages, write, pages);
> > > +	if (npinned != npages)
> > > +		goto err;
> > > +
> > As I said I have doubts about the whole approach, but this
> > implementation in particular isn't a good idea
> > as it keeps the page around forever.
> > So no THP, no NUMA rebalancing,
> 
> 
> This is the price of all GUP users not only vhost itself.

Yes. GUP is just not a great interface for vhost to use.

> What's more
> important, the goal is not to be left too much behind for other backends
> like DPDK or AF_XDP (all of which are using GUP).

So these guys assume userspace knows what it's doing.
We can't assume that.

> 
> > userspace-controlled
> > amount of memory locked up and not accounted for.
> 
> 
> It's pretty easy to add this since the slow path was still kept. If we
> exceeds the limitation, we can switch back to slow path.
> 
> > 
> > Don't get me wrong it's a great patch in an ideal world.
> > But then in an ideal world no barriers smap etc are necessary at all.
> 
> 
> Again, this is only for metadata accessing not the data which has been used
> for years for real use cases.
> 
> For SMAP, it makes senses for the address that kernel can not forcast. But
> it's not the case for the vhost metadata since we know the address will be
> accessed very frequently. For speculation barrier, it helps nothing for the
> data path of vhost which is a kthread.

I don't see how a kthread makes any difference. We do have a validation
step which makes some difference.

> Packet or AF_XDP benefit from
> accessing metadata directly, we should do it as well.
> 
> Thanks