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Subject: Re: [virtio-comment] [PATCH v2] virtio-net: support distinguishing between partial and full checksum



å 2023/11/1 äå12:16, Jason Wang åé:

On Sat, Oct 28, 2023 at 10:36âAM Heng Qi <hengqi@linux.alibaba.com> wrote:



å 2023/10/27 äå3:39, Michael S. Tsirkin åé:

On Thu, Oct 19, 2023 at 02:17:20PM +0800, Heng Qi wrote:

virtio-net works in a virtualized system and is somewhat different from
physical nics. One of the differences is that to save virtio device
resources, rx may receive packets with partial checksum. However, XDP may
cause partially checksummed packets to be dropped. So XDP loading conflicts
with the feature VIRTIO_NET_F_GUEST_CSUM.

This patch lets the device to supply fully checksummed packets to the driver.
Then XDP can coexist with VIRTIO_NET_F_GUEST_CSUM to enjoy the benefits of
device verification checksum.

In addition, implementation of some performant devices do not generate
partially checksummed packets, but the standard driver still need to clear
VIRTIO_NET_F_GUEST_CSUM when loading XDP. If these devices enable the
full checksum offloading, then the driver can load XDP without clearing
VIRTIO_NET_F_GUEST_CSUM.

A new feature bit VIRTIO_NET_F_GUEST_FULL_CSUM is added to solve the above
situation, which provides the driver with configurable receive full checksum
offload. If the offload is enabled, then the device must supply fully
checksummed packets to the driver.

Use case example:
If VIRTIO_NET_F_GUEST_FULL_CSUM is negotiated and receive full checksum
offload is enabled, after XDP processes a packet with full checksum, the
VIRTIO_NET_HDR_F_DATA_VALID bit is still retained, resulting in the stack
not needing to validate the checksum again. This is useful for guests:
    1. Bring the driver advantages such as cpu savings.
    2. For devices that do not generate partially checksummed packets themselves,
       XDP can be loaded in the driver without modifying the hardware behavior.

Several solutions have been discussed in the previous proposal[1].
After historical discussion, we have tried the method proposed by Jason[2],
but some complex scenarios and challenges are difficult to deal with.
We now return to the method suggested in [1].

[1] https://lists.oasis-open.org/archives/virtio-dev/202305/msg00291.html
[2] https://lore.kernel.org/all/20230628030506.2213-1-hengqi@linux.alibaba.com/

Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
v1->v2:
      1. Modify full checksum functionality as a configurable offload
         that is initially turned off. @Jason

   device-types/net/description.tex | 54 ++++++++++++++++++++++++++++----
   1 file changed, 48 insertions(+), 6 deletions(-)

diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index 76585b0..3c34f27 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -88,6 +88,8 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
   \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
       channel.

+\item[VIRTIO_NET_F_GUEST_FULL_CSUM (50)] Driver handles packets with full checksum.
+
   \item[VIRTIO_NET_F_HASH_TUNNEL(51)] Device supports inner header hash for encapsulated packets.

   \item[VIRTIO_NET_F_VQ_NOTF_COAL(52)] Device supports virtqueue notification coalescing.
@@ -133,6 +135,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
   \item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM.
   \item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM.
   \item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM.
+\item[VIRTIO_NET_F_GUEST_FULL_CSUM] Requires VIRTIO_NET_F_GUEST_CSUM and VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.

   \item[VIRTIO_NET_F_HOST_TSO4] Requires VIRTIO_NET_F_CSUM.
   \item[VIRTIO_NET_F_HOST_TSO6] Requires VIRTIO_NET_F_CSUM.

What about all of these:

device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_TSO4] Requires VIRTIO_NET_F_GUEST_CSUM.
device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_TSO6] Requires VIRTIO_NET_F_GUEST_CSUM.
device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM.
device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM.
device-types/net/description.tex:\item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM.



can TSO/UFO/USO work with VIRTIO_NET_F_GUEST_FULL_CSUM as opposed to VIRTIO_NET_F_GUEST_CSUM?

Both GUEST_FULL_CSUM and GUEST_CSUM can work with GUEST_TSO/USO/UFO.

Yes. For software devices I guess it will have a lot of performance
penalty. So it should be disabled by default anyhow. The idea is to
delay the csum as late as possible.


Yes. I totally agree.




Their important difference is that if GUEST_CSUM is negotiated, the
driver can handle partial checksum.




@@ -390,6 +393,13 @@ \subsection{Device Initialization}\label{sec:Device Types / Network Device / Dev
   \ref{sec:Device Types / Network Device / Device Operation /
   Processing of Incoming Packets}~\nameref{sec:Device Types /
   Network Device / Device Operation / Processing of Incoming Packets} below.
+
+\item The VIRTIO_NET_F_GUEST_FULL_CSUM feature indicates that the driver handles
+  packets with full checksum and does not handle packets with partial checksum,

So we need to change definition of VIRTIO_NET_F_GUEST_CSUM then.


Also this is not exactly right. As defined driver must be able to handle
partial checksum too.


How about this:

- change definition above to just "Driver handles packets with full checksum."

- if VIRTIO_NET_F_GUEST_FULL_CSUM is set but VIRTIO_NET_F_GUEST_CSUM is
    clear driver requires full checksum

- if VIRTIO_NET_F_GUEST_FULL_CSUM is clear but VIRTIO_NET_F_GUEST_CSUM is
    set driver supports partial checksum

- if VIRTIO_NET_F_GUEST_FULL_CSUM and VIRTIO_NET_F_GUEST_CSUM are
    set then the behavior is as you describe: VIRTIO_NET_F_GUEST_CSUM
    takes preference, but you can disable it with VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
    if that is supported.

Jason wanted this feature to be enabled only when XDP is loading,
and this is the context in which this patch was proposed.

How do you pay attention to this?

I don't see any conflict, or anything I miss?


Yes, our request was met.

If GUEST_FULL_CSUM and GUEST_CSUM are independent,
that is, GUEST_FULL_CSUM can be successfully validated without GUEST_CSUM.
Then we need to re-describe most of the existing behavior of GUEST_CSUM for FULL_CSUM in the spec, this part is overlapping. Moreover, the relationship between FULL_CSUM 
and GUEST_CSUM also needs to be processed in the full text.

So I think it seems clearer to constrain the behavior of GUEST_CSUM by
treating FULL_CSUM as a subset of GUEST_CSUM.

For example we don't need to make the following changes:
\item[VIRTIO_NET_F_GUEST_TSO4] Requires VIRTIO_NET_F_GUEST_CSUM or VIRTIO_NET_F_GUEST_FULL_CSUM. \item[VIRTIO_NET_F_GUEST_TSO6] Requires VIRTIO_NET_F_GUEST_CSUM or VIRTIO_NET_F_GUEST_FULL_CSUM. \item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM or VIRTIO_NET_F_GUEST_FULL_CSUM. \item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM or VIRTIO_NET_F_GUEST_FULL_CSUM. \item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM or VIRTIO_NET_F_GUEST_FULL_CSUM. 

Thanks!



Thanks

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