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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 26 Dec 2018 10:02:54 -0500

On Wed, Dec 26, 2018 at 11:57:32AM +0800, Jason Wang wrote:
> 
> On 2018/12/25 下午8:50, Michael S. Tsirkin wrote:
> > On Tue, Dec 25, 2018 at 06:05:25PM +0800, Jason Wang wrote:
> > > On 2018/12/25 上午2:10, Michael S. Tsirkin wrote:
> > > > On Mon, Dec 24, 2018 at 03:53:16PM +0800, Jason Wang wrote:
> > > > > On 2018/12/14 下午8:36, Michael S. Tsirkin wrote:
> > > > > > 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.
> > > > > The pages wil be released during set features.
> > > > > 
> > > > > 
> > > > > > > > So no THP, no NUMA rebalancing,
> > > > > For THP, we will probably miss 2 or 4 pages, but does this really matter
> > > > > consider the gain we have?
> > > > We as in vhost? networking isn't the only thing guest does.
> > > > We don't even know if this guest does a lot of networking.
> > > > You don't
> > > > know what else is in this huge page. Can be something very important
> > > > that guest touches all the time.
> > > 
> > > Well, the probability should be very small consider we usually give several
> > > gigabytes to guest. The rest of the pages that doesn't sit in the same
> > > hugepage with metadata can still be merged by THP.  Anyway, I can test the
> > > differences.
> > Thanks!
> > 
> > > > > For NUMA rebalancing, I'm even not quite sure if
> > > > > it can helps for the case of IPC (vhost). It looks to me the worst case it
> > > > > may cause page to be thrash between nodes if vhost and userspace are running
> > > > > in two nodes.
> > > > So again it's a gain for vhost but has a completely unpredictable effect on
> > > > other functionality of the guest.
> > > > 
> > > > That's what bothers me with this approach.
> > > 
> > > So:
> > > 
> > > - The rest of the pages could still be balanced to other nodes, no?
> > > 
> > > - try to balance metadata pages (belongs to co-operate processes) itself is
> > > still questionable
> > I am not sure why. It should be easy enough to force the VCPU and vhost
> > to move (e.g. start them pinned to 1 cpu, then pin them to another one).
> > Clearly sometimes this would be necessary for load balancing reasons.
> 
> 
> Yes, but it looks to me the part of motivation of auto NUMA is to avoid
> manual pinning.

... of memory. Yes.

> 
> > With autonuma after a while (could take seconds but it will happen) the
> > memory will migrate.
> > 
> 
> Yes. As you mentioned during the discuss, I wonder we could do it similarly
> through mmu notifier like APIC access page in commit c24ae0dcd3e ("kvm: x86:
> Unpin and remove kvm_arch->apic_access_page")

That would be a possible approach.

> 
> > 
> > 
> > > > 
> > > > 
> > > > 
> > > > > > > This is the price of all GUP users not only vhost itself.
> > > > > > Yes. GUP is just not a great interface for vhost to use.
> > > > > Zerocopy codes (enabled by defualt) use them for years.
> > > > But only for TX and temporarily. We pin, read, unpin.
> > > 
> > > Probably not. For several reasons that the page will be not be released soon
> > > or held for a very long period of time or even forever.
> > 
> > With zero copy? Well it's pinned until transmit. Takes a while
> > but could be enough for autocopy to work esp since
> > its the packet memory so not reused immediately.
> > 
> > > > Your patch is different
> > > > 
> > > > - it writes into memory and GUP has known issues with file
> > > >     backed memory
> > > 
> > > The ordinary user for vhost is anonymous pages I think?
> > 
> > It's not the most common scenario and not the fastest one
> > (e.g. THP does not work) but file backed is useful sometimes.
> > It would not be nice at all to corrupt guest memory in that case.
> 
> 
> Ok.
> 
> 
> > 
> > > > - it keeps pages pinned forever
> > > > 
> > > > 
> > > > 
> > > > > > > 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.
> > > > > What kind of assumption do you they have?
> > > > > 
> > > > > 
> > > > > > > > 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.
> > > > > The problem is not kthread but the address of userspace address. The
> > > > > addresses of vq metadata tends to be consistent for a while, and vhost knows
> > > > > they will be frequently. SMAP doesn't help too much in this case.
> > > > > 
> > > > > Thanks.
> > > > It's true for a real life applications but a malicious one
> > > > can call the setup ioctls any number of times. And SMAP is
> > > > all about malcious applications.
> > > 
> > > We don't do this in the path of ioctl, there's no context switch between
> > > userspace and kernel in the worker thread. SMAP is used to prevent kernel
> > > from accessing userspace pages unexpectedly which is not the case for
> > > metadata access.
> > > 
> > > Thanks
> > OK let's forget smap for now.
> 
> 
> Some numbers I measured:
> 
> On an old Sandy bridge machine without SMAP support. Remove speculation
> barrier boost the performance from 4.6Mpps to 5.1Mpps
> 
> On a newer Broadwell machine with SMAP support. Remove speculation barrier
> only gives 2%-5% improvement, disable SMAP completely through Kconfig boost
> 57% performance from 4.8Mpps to 7.5Mpps. (Vmap gives 6Mpps - 6.1Mpps, it
> only bypass SMAP for metadata).
> 
> So it looks like for recent machine, SMAP becomes pain point when the copy
> is short (e.g 64B) for high PPS.
> 
> Thanks

Thanks a lot for looking into this!

So first of all users can just boot with nosmap, right?
What's wrong with that? Yes it's not fine-grained but OTOH
it's easy to understand.

And I guess this confirms that if we are going to worry
about smap enabled, we need to look into packet copies
too, not just meta-data.

Vaguely could see a module option (off by default)
where vhost basically does user_access_begin
when it starts running, then uses unsafe accesses
in vhost and tun and then user_access_end.

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