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Subject: Re: [virtio-comment] Re: [PATCH v4 1/2] virtio-net: update description for VIRTIO_NET_F_GUEST_CSUM.



å 2023/12/5 äå11:52, Jason Wang åé:

On Mon, Dec 4, 2023 at 5:34âPM Heng Qi <hengqi@linux.alibaba.com> wrote:



å 2023/12/4 äå5:05, Michael S. Tsirkin åé:

On Mon, Dec 04, 2023 at 04:59:49PM +0800, Jason Wang wrote:

On Mon, Dec 4, 2023 at 4:53âPM Michael S. Tsirkin <mst@redhat.com> wrote:

On Mon, Dec 04, 2023 at 04:49:46PM +0800, Jason Wang wrote:

On Mon, Dec 4, 2023 at 3:37âPM Heng Qi <hengqi@linux.alibaba.com> wrote:


å 2023/12/4 äå3:18, Jason Wang åé:

On Fri, Dec 1, 2023 at 3:16âPM Heng Qi <hengqi@linux.alibaba.com> wrote:

å 2023/12/1 äå3:05, Jason Wang åé:

On Fri, Dec 1, 2023 at 2:30âPM Heng Qi <hengqi@linux.alibaba.com> wrote:

å 2023/12/1 äå2:24, Heng Qi åé:

å 2023/12/1 äå1:18, Jason Wang åé:

On Wed, Nov 29, 2023 at 4:23âPM Heng Qi <hengqi@linux.alibaba.com>
wrote:

å 2023/11/29 äå4:00, Jason Wang åé:

On Tue, Nov 28, 2023 at 4:08âPM Heng Qi <hengqi@linux.alibaba.com>
wrote:

To prevent readers from misunderstanding that the driver can
only handles packets with partial checksum when
VIRTIO_NET_F_GUEST_CSUM is negotiated, we update the description.

Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
---
       device-types/net/description.tex | 2 +-
       1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/device-types/net/description.tex
b/device-types/net/description.tex
index aff5e08..529f470 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -38,7 +38,7 @@ \subsection{Feature bits}\label{sec:Device Types
/ Network Device / Feature bits
       \begin{description}
       \item[VIRTIO_NET_F_CSUM (0)] Device handles packets with
partial checksum offload.

-\item[VIRTIO_NET_F_GUEST_CSUM (1)] Driver handles packets with
partial checksum.
+\item[VIRTIO_NET_F_GUEST_CSUM (1)] Driver handles packets with
partial checksum or full checksum.

So patch 2 said

"
+\item[VIRTIO_NET_F_GUEST_FULL_CSUM (64)] Driver handles packets with
full checksum.
       \end{description}
"

Is there any difference between the two "full checksum" here?

There's no difference.

The core is that VIRTIO_NET_F_GUEST_FULL_CSUM means that the driver
"can
only" handle packets with full checksum.

This seems to be odd.

Driver can always handle packet with full checksum, no?

Yes.


I meant it
will be then to be functional equivalent to !
VIRTIO_NET_F_GUEST_FULL_CSUM?

Are you referring to
"functional equivalent to !VIRTIO_NET_F_GUEST_CSUM" ?

Sorry, this is a typo. I meant

Are you referring to
"functional equivalent to !VIRTIO_NET_F_GUEST_FULL_CSUM" ?


If so, I think it's no.

Maybe a description similar to the following would be more clearer:

+\item[VIRTIO_NET_F_GUEST_FULL_CSUM (64)] Driver does not handle
packets with partial checksum.

I may miss something here, but what's the difference between

VIRTIO_NET_F_GUEST_FULL_CSUM

and

!VIRTIO_NET_F_GUEST_CSUM?

    From the device perspective:

If !VIRTIO_NET_F_GUEST_CSUM, the device delivers packets with full
checksum to the driver,
but the device can not validate the checksum for these packets. That is,
the flags in virtio-net-hdr
will not contain _DATA_VALID, and the driver or stack needs to validate
these packets.

If VIRTIO_NET_F_GUEST_FULL_CSUM, the device delivers packets with full
checksum to the driver,
and the device can validate the checksum for these packets. That is, the
flags in virtio-net-hdr
will contain _DATA_VALID,

I think DATA_VALID is optional here as device can't recognize all type
of protocols.

Yes, you are right, so I used "device *can*" here. Which packet types
the device recognizes or validates
depends on the device's implementation. This is also the current
practice of GUEST_CSUM.


and the driver or stack does not need to
validate these packets.

Ok, so I think there're something that is subtle here,

Ok, I see.


and that's why
I'm asking here:

1) "Driver does not handle packets with partial checksum" is not
accurate, !GUEST_CUSM also fit for this definition.
2) "Driver handles packets with full checksum" is kind of ambiguous as
it doesn't say whether or not the packet has been validated or not.

Maybe the description below would be less subtle?
+\item[VIRTIO_NET_F_GUEST_CSUM (1)] Driver handles packets with partial
checksum or full checksum.

I'd suggest to leave it as is. As I didn't find any issue since even
with DATA_VALID. Did you?


+\item[VIRTIO_NET_F_GUEST_FULL_CSUM (64)] The driver handles packets
with full checksum,
and the device optionally validates the packet's checksum.

Or maybe something like (not a native speaker)

The driver handles packets with full checksum which the device has
already validated.

Thanks

I feel we just need a proper definition of what does "full checksum"
mean in this context. It is used but not defined.
Assume this feature was negotiated.
My understanding is that this is just like VIRTIO_NET_F_GUEST_CSUM
but certain values in the header are then disallowed? Which?
This should be in the spec.

Yes, I think it is probably the headers that DATA_VALID can work. We
never define it in the past.

E.g in the Linux we map DATA_VALID to CHECKSUM_UNNECESSARY, but it can
only work for some specific protocols:

"""
   *   %CHECKSUM_UNNECESSARY is applicable to following protocols:
   *
   *     - TCP: IPv6 and IPv4.
   *     - UDP: IPv4 and IPv6. A device may apply CHECKSUM_UNNECESSARY to a
   *       zero UDP checksum for either IPv4 or IPv6, the networking stack
   *       may perform further validation in this case.
   *     - GRE: only if the checksum is present in the header.
   *     - SCTP: indicates the CRC in SCTP header has been validated.
   *     - FCOE: indicates the CRC in FC frame has been validated.
"""

I'm not sure whether it's just fine to duplicate the definition or
it's too late to define any now.

I think it's mostly harmless for other protocols.

I'm not sure if this should be defined by a new FULL_CSUM feature.
This seems to be an issue with GUEST_CSUM.

I think we should supplement these with a new patch for GUEST_CSUM?

Probably. My understanding is:

You want to reuse DATA_VALID here, so we need to stick to a consistent
semantic for GUEST_CUSM and FULL_CSUM. So we need a definition of
"full csum" or what kind of packet could DATA_VALID work here.


I agree, we can be clear about what types of packets DATA_VALID might
cover, e.g. TCP/UDP/GRE/SCTP/FoCE.

But I think we also need something like \field{supported_validate_types} to
indicate which packet types the device supports validating and setting DATA_VALID, otherwise the device driver that negotiates this feature may fail to live migration. 
Am I right?
I'm not sure how GUEST_CSUM works now as it should also suffer from the above mentioned issues with live migration, but no devices are reporting this right now.
Maybe, each device only supports checksum verification for TCP/UDP by default? I don't know. But I hope we can focus on this and get consensus, because our hw release date is coming soon. 

Thanks a lot!



Thanks


Thanks!


After this is written up we will come up with a good short
description for the feature bit.


Thanks


Thanks!


Thanks


Thanks!


Thanks


Thanks!


Thanks


Thanks!


Thanks


\item[VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2)] Control channel offloads
               reconfiguration support.
--
2.19.1.6.gb485710b


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