Re: Designing a safe RX-zero-copy Memory Model for Networking

On Mon, 12 Dec 2016 06:49:03 -0800
John Fastabend [off-list ref] wrote:

On 16-12-12 06:14 AM, Mike Rapoport wrote:

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On Mon, Dec 12, 2016 at 10:40:42AM +0100, Jesper Dangaard Brouer wrote:  

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On Mon, 12 Dec 2016 10:38:13 +0200 Mike Rapoport [off-list ref] wrote:
 

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Hello Jesper,

On Mon, Dec 05, 2016 at 03:31:32PM +0100, Jesper Dangaard Brouer wrote:  

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Hi all,

This is my design for how to safely handle RX zero-copy in the network
stack, by using page_pool[1] and modifying NIC drivers.  Safely means
not leaking kernel info in pages mapped to userspace and resilience
so a malicious userspace app cannot crash the kernel.

Design target
=============

Allow the NIC to function as a normal Linux NIC and be shared in a
safe manor, between the kernel network stack and an accelerated
userspace application using RX zero-copy delivery.

Target is to provide the basis for building RX zero-copy solutions in
a memory safe manor.  An efficient communication channel for userspace
delivery is out of scope for this document, but OOM considerations are
discussed below (`Userspace delivery and OOM`_).    

Sorry, if this reply is a bit off-topic.  

It is very much on topic IMHO :-)
 

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I'm working on implementation of RX zero-copy for virtio and I've dedicated
some thought about making guest memory available for physical NIC DMAs.
I believe this is quite related to your page_pool proposal, at least from
the NIC driver perspective, so I'd like to share some thoughts here.  

Seems quite related. I'm very interested in cooperating with you! I'm
not very familiar with virtio, and how packets/pages gets channeled
into virtio.  

They are copied :-)
Presuming we are dealing only with vhost backend, the received skb
eventually gets converted to IOVs, which in turn are copied to the guest
memory. The IOVs point to the guest memory that is allocated by virtio-net
running in the guest.
  

Great I'm also doing something similar.

My plan was to embed the zero copy as an AF_PACKET mode and then push
a AF_PACKET backend into vhost. I'll post a patch later this week.

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The idea is to dedicate one (or more) of the NIC's queues to a VM, e.g.
using macvtap, and then propagate guest RX memory allocations to the NIC
using something like new .ndo_set_rx_buffers method.  

I believe the page_pool API/design aligns with this idea/use-case.
 

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What is your view about interface between the page_pool and the NIC
drivers?  

In my Prove-of-Concept implementation, the NIC driver (mlx5) register
a page_pool per RX queue.  This is done for two reasons (1) performance
and (2) for supporting use-cases where only one single RX-ring queue is
(re)configured to support RX-zero-copy.  There are some associated
extra cost of enabling this mode, thus it makes sense to only enable it
when needed.

I've not decided how this gets enabled, maybe some new driver NDO.  It
could also happen when a XDP program gets loaded, which request this
feature.

The macvtap solution is nice and we should support it, but it requires
VM to have their MAC-addr registered on the physical switch.  This
design is about adding flexibility. Registering an XDP eBPF filter
provides the maximum flexibility for matching the destination VM.  

I'm not very familiar with XDP eBPF, and it's difficult for me to estimate
what needs to be done in BPF program to do proper conversion of skb to the
virtio descriptors.  

I don't think XDP has much to do with this code and they should be done
separately. XDP runs eBPF code on received packets after the DMA engine
has already placed the packet in memory so its too late in the process.

It does not have to be connected to XDP.  My idea should support RX
zero-copy into normal sockets, without XDP.

My idea was to pre-VMA map the RX ring, when zero-copy is requested,
thus it is not too late in the process.  When frame travel the normal
network stack, then require the SKB-read-only-page mode (skb-frags).
If the SKB reach a socket that support zero-copy, then we can do RX
zero-copy on normal sockets.

 

The other piece here is enabling XDP in vhost but that is again separate
IMO.

Notice that ixgbe supports pushing packets into a macvlan via 'tc'
traffic steering commands so even though macvlan gets an L2 address it
doesn't mean it can't use other criteria to steer traffic to it.

This sounds interesting. As this allow much more flexibility macvlan
matching, which I like, but still depending on HW support. 

 

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We were not considered using XDP yet, so we've decided to limit the initial
implementation to macvtap because we can ensure correspondence between a
NIC queue and virtual NIC, which is not the case with more generic tap
device. It could be that use of XDP will allow for a generic solution for
virtio case as well.  

Interesting this was one of the original ideas behind the macvlan
offload mode. iirc Vlad also was interested in this.

I'm guessing this was used because of the ability to push macvlan onto
its own queue?

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Have you considered using "push" model for setting the NIC's RX memory?  

I don't understand what you mean by a "push" model?  

Currently, memory allocation in NIC drivers boils down to alloc_page with
some wrapping code. I see two possible ways to make NIC use of some
preallocated pages: either NIC driver will call an API (probably different
from alloc_page) to obtain that memory, or there will be NDO API that
allows to set the NIC's RX buffers. I named the later case "push".  

I prefer the ndo op. This matches up well with AF_PACKET model where we
have "slots" and offload is just a transparent "push" of these "slots"
to the driver. Below we have a snippet of our proposed API,

Hmmm. If you can rely on hardware setup to give you steering and
dedicated access to the RX rings.  In those cases, I guess, the "push"
model could be a more direct API approach.

I was shooting for a model that worked without hardware support.  And
then transparently benefit from HW support by configuring a HW filter
into a specific RX queue and attaching/using to that queue.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

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