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

From: Jason Wang
Date: Thu Dec 27 2018 - 04:40:09 EST

On 2018/12/26 äå11:02, Michael S. Tsirkin wrote:
On Wed, Dec 26, 2018 at 11:57:32AM +0800, Jason Wang wrote:
On 2018/12/25 äå8:50, Michael S. Tsirkin wrote:
On Tue, Dec 25, 2018 at 06:05:25PM +0800, Jason Wang wrote:
On 2018/12/25 äå2:10, Michael S. Tsirkin wrote:
On Mon, Dec 24, 2018 at 03:53:16PM +0800, Jason Wang wrote:
On 2018/12/14 äå8:36, Michael S. Tsirkin wrote:
On Fri, Dec 14, 2018 at 11:57:35AM +0800, Jason Wang wrote:
On 2018/12/13 äå11:44, 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<jasowang@xxxxxxxxxx>
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));
vhost_vq_reset(dev, vq);
if (vq->handle_kick)
@@ -614,6 +617,102 @@ static void vhost_clear_msg(struct vhost_dev *dev)
+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

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.

- 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.

Yes, this looks possible, and the conversion seems not hard. Let me have a try with this.


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.

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.

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 a lot for looking into this!

So first of all users can just boot with nosmap, right?
What's wrong with that?

Nothing wrong, just realize we had this kernel parameter.

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.

For packet copies, we can do batch copy which is pretty simple for the case of XDP. I've already had patches for this.

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.

Using user_access_begin() is more tricky than imaged. E.g it requires:

- userspace address to be validated before through access_ok() [1]

- It doesn't support calling a function that does explicit schedule since SMAP/PAN state is not maintained through schedule() [2]



So calling user_access_begin() all the time when vhost is running seems pretty dangerous.

For a better batched datacopy, I tend to build not only XDP but also skb in vhost in the future.


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