virtio-comment

There are few small issues around size of receive buffer and size of the
struct virtio_net_hdr as following.

1. VIRTIO_NET_F_HASH_REPORT is only applicable on the rx side processing.
However it changes the size of the struct virtio_net_hdr for the tx side
as well. This was not clarified when VIRTIO_NET_F_HASH_REPORT feature was
introduced.

2. Receive buffer size calculation description and requirements did not
reflect negotiation of VIRTIO_NET_F_HASH_REPORT.

3. Receive buffer size calculation was incorrect for ipv6 guest gso feature.

Above issue 2 was reported in [1].
Since issue 2 and 3 both touches the same set of requirements and description,
they are updated under a new issue along with tx side fix as well touching the
clarification needed for VIRTIO_NET_F_HASH_REPORT.

Hence above three issues are fixed under issue of [2].

Patch summary:
patch-1 fixes receive side description and requirement
patch-2 adds transmit side clarification for VIRTIO_NET_F_HASH_REPORT

changelog:
v3->v4:
- addressed comments from Cornelia
- rephrase for virtio_net_hdr size calculation
v2->v3:
- addressed comments from Cornelia and Xuan
- added virtio_net_hdr inclusion description in buffer size calculation text
- rephrase tx to refer to the rx side virtio_net_hdr consideration
- addressed comments from Xuan
- replaced 'field' with 'structure'
v1->v2:
- addressed comments from Cornelia
- rephrase buffer size wording without explicit length of virtio_net_hdr
at 3 places.
- replaced otherwise to ', otherwise'
- rephrase non normative to write as driver normative
v0->v1:
- addressed comments from Xuan to rewrite tx normative


Parav Pandit (2):
virtio-net: Fix receive buffer size calculation text
virtio-net: Clarify the size of the struct virtio_net_hdr for tx

device-types/net/description.tex | 29 ++++++++++++++++++++++-------
1 file changed, 22 insertions(+), 7 deletions(-)

--
2.34.1

Receive buffer size calculation is based on the following
negotiated features.

The text has wrong calculation for IPv6 and also it has missed
VIRTIO_NET_F_HASH_REPORT.

The problem of igorance of VIRTIO_NET_F_HASH_REPORT is reported
in [1], however fix for ipv6 payload length must also be
considered.

Since for the both the fixes touching same requirements, a
new issue is created as [2].

This patch brings following fixes.

1. Fix annotating struct virtio_net_hdr as field
2. Fix receive buffer calculation for guest GSO cases to consider
ipv6 payload length
3. small grammar corrections for article
4. reword the requirement to consider the virtio_ndr_hdr which is
depends on the negotiated feature, hence first clarify the
struct virtio_net_hdr size

[1] https://github.com/oasis-tcs/virtio-spec/issues/170
[2] https://github.com/oasis-tcs/virtio-spec/issues/183

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/170
Fixes: https://github.com/oasis-tcs/virtio-spec/issues/183
Signed-off-by: Parav Pandit <parav@nvidia.com>

---
changelog:
v3->v4:
- addressed comments from Cornelia
- rephrase for virtio_net_hdr size calculation
v2->v3:
- addressed comments from Cornelia
- added virtio_net_hdr inclusion description in buffer size calculation text
v1->v2:
- addressed comments from Cornelia
- rephrase buffer size wording without explicit length of virtio_net_hdr
at 3 places.
- replaced otherwise to ', otherwise'
- rephrase non normative to write as driver normative
---
device-types/net/description.tex | 26 +++++++++++++++++++-------
1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index aff5e08..17e33fa 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -657,10 +657,15 @@ \subsubsection{Setting Up Receive Buffers}\label{sec:Device Types / Network Devi
If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
VIRTIO_NET_F_GUEST_UFO, VIRTIO_NET_F_GUEST_USO4 or VIRTIO_NET_F_GUEST_USO6
features are used, the maximum incoming packet
-will be to 65550 bytes long (the maximum size of a
-TCP or UDP packet, plus the 14 byte ethernet header), otherwise
-1514 bytes. The 12-byte struct virtio_net_hdr is prepended to this,
-making for 65562 or 1526 bytes.
+will be 65589 bytes long (14 bytes of Ethernet header, plus 40 bytes of
+the IPv6 header, plus 65535 bytes of maximum IPv6 payload including any
+extension header), otherwise 1514 bytes.
+When VIRTIO_NET_F_HASH_REPORT is not negotiated, the required receive buffer
+size is either 65601 or 1526 bytes accounting for 20 bytes of
+\field{struct virtio_net_hdr} followed by receive packet.
+When VIRTIO_NET_F_HASH_REPORT is negotiated, the required receive buffer
+size is either 65609 or 1534 bytes accounting for 12 bytes of
+\field{struct virtio_net_hdr} followed by receive packet.

\drivernormative{\paragraph}{Setting Up Receive Buffers}{Device Types / Network Device / Device Operation / Setting Up Receive Buffers}

@@ -669,18 +674,25 @@ \subsubsection{Setting Up Receive Buffers}\label{sec:Device Types / Network Devi
\begin{itemize}
\item If VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, VIRTIO_NET_F_GUEST_UFO,
VIRTIO_NET_F_GUEST_USO4 or VIRTIO_NET_F_GUEST_USO6 are negotiated, the driver SHOULD populate
- the receive queue(s) with buffers of at least 65562 bytes.
+ the receive queue(s) with buffers of at least 65609 bytes if
+ VIRTIO_NET_F_HASH_REPORT is negotiated, and of at least 65601 bytes if not.
\item Otherwise, the driver SHOULD populate the receive queue(s)
- with buffers of at least 1526 bytes.
+ with buffers of at least 1534 bytes if VIRTIO_NET_F_HASH_REPORT
+ is negotiated, and of at least 1526 bytes if not.
\end{itemize}
\item If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer MUST be at
-least the size of the struct virtio_net_hdr.
+least size of \field{struct virtio_net_hdr},
+i.e. 20 bytes if VIRTIO_NET_F_HASH_REPORT is negotiated, and 12 bytes if not.
\end{itemize}

\begin{note}
Obviously each buffer can be split across multiple descriptor elements.
\end{note}

+When calculating the size of \field{struct virtio_net_hdr}, the driver
+must consider all the fields inclusive up to \field{padding_reserved},
+i.e. 20 bytes if VIRTIO_NET_F_HASH_REPORT is negotiated, and 12 bytes if not.
+
If VIRTIO_NET_F_MQ is negotiated, each of receiveq1\ldots receiveqN
that will be used SHOULD be populated with receive buffers.

--
2.34.1

The feature VIRTIO_NET_F_HASH_REPORT only applies to the receive side.
However, when VIRTIO_NET_F_HASH_REPORT feature was introduced, it was
not clarified that the size of the struct virtio_net_hdr on the packet
transmission also uses higher size when VIRTIO_NET_F_HASH_REPORT is
negotiated.

Explicitly clarify this.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/183
Signed-off-by: Parav Pandit <parav@nvidia.com>

---
changelog:
v2->v3:
- rephrase tx to refer to the rx side virtio_net_hdr consideration
- addressed comments from Xuan
- replaced 'field' with 'structure'
v0->v1:
- addressed comments from Xuan to rewrite tx normative
---
device-types/net/description.tex | 3 +++
1 file changed, 3 insertions(+)

diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index 17e33fa..0fd7833 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -528,6 +528,9 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De

\drivernormative{\paragraph}{Packet Transmission}{Device Types / Network Device / Device Operation / Packet Transmission}

+For the transmit packet buffer, the driver MUST use the size of the
+structure \field{struct virtio_net_hdr} same as the receive packet buffer.
+
The driver MUST set \field{num_buffers} to zero.

If VIRTIO_NET_F_CSUM is not negotiated, the driver MUST set
--
2.34.1