Jiri Pirko
2023-Mar-09 09:45 UTC
[patch net-next v4] net: virtio_net: implement exact header length guest feature
From: Jiri Pirko <jiri at nvidia.com>
Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
set implicates that device benefits from knowing the exact size
of the header. For compatibility, to signal to the device that
the header is reliable driver also needs to set this feature.
Without this feature set by driver, device has to figure
out the header size itself.
Quoting the original virtio spec:
"hdr_len is a hint to the device as to how much of the header needs to
be kept to copy into each packet"
"a hint" might not be clear for the reader what does it mean, if it is
"maybe like that" of "exactly like that". This feature just
makes it
crystal clear and let the device count on the hdr_len being filled up
by the exact length of header.
Also note the spec already has following note about hdr_len:
"Due to various bugs in implementations, this field is not useful
as a guarantee of the transport header size."
Without this feature the device needs to parse the header in core
data path handling. Accurate information helps the device to eliminate
such header parsing and directly use the hardware accelerators
for GSO operation.
virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
The driver already complies to fill the correct value. Introduce the
feature and advertise it.
Note that virtio spec also includes following note for device
implementation:
"Caution should be taken by the implementation so as to prevent
a malicious driver from attacking the device by setting
an incorrect hdr_len."
There is a plan to support this feature in our emulated device.
A device of SolidRun offers this feature bit. They claim this feature
will save the device a few cycles for every GSO packet.
Link:
https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.html#x1-230006x3
Signed-off-by: Jiri Pirko <jiri at nvidia.com>
Reviewed-by: Parav Pandit <parav at nvidia.com>
Reviewed-by: Alvaro Karsz <alvaro.karsz at solid-run.com>
Acked-by: Michael S. Tsirkin <mst at redhat.com>
Acked-by: Willem de Bruijn <willemb at google.com>
---
v3->v4:
- fixed double "which when"
v2->v3:
- changed the first paragraph in patch description according to
Michael's suggestion
- added Link tag with link to the spec
v1->v2:
- extended patch description
---
drivers/net/virtio_net.c | 6 ++++--
include/uapi/linux/virtio_net.h | 1 +
2 files changed, 5 insertions(+), 2 deletions(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index fb5e68ed3ec2..e85b03988733 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -62,7 +62,8 @@ static const unsigned long guest_offloads[] = {
VIRTIO_NET_F_GUEST_UFO,
VIRTIO_NET_F_GUEST_CSUM,
VIRTIO_NET_F_GUEST_USO4,
- VIRTIO_NET_F_GUEST_USO6
+ VIRTIO_NET_F_GUEST_USO6,
+ VIRTIO_NET_F_GUEST_HDRLEN
};
#define GUEST_OFFLOAD_GRO_HW_MASK ((1ULL << VIRTIO_NET_F_GUEST_TSO4) | \
@@ -4213,7 +4214,8 @@ static struct virtio_device_id id_table[] = {
VIRTIO_NET_F_CTRL_MAC_ADDR, \
VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
VIRTIO_NET_F_SPEED_DUPLEX, VIRTIO_NET_F_STANDBY, \
- VIRTIO_NET_F_RSS, VIRTIO_NET_F_HASH_REPORT, VIRTIO_NET_F_NOTF_COAL
+ VIRTIO_NET_F_RSS, VIRTIO_NET_F_HASH_REPORT, VIRTIO_NET_F_NOTF_COAL, \
+ VIRTIO_NET_F_GUEST_HDRLEN
static unsigned int features[] = {
VIRTNET_FEATURES,
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index b4062bed186a..12c1c9699935 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -61,6 +61,7 @@
#define VIRTIO_NET_F_GUEST_USO6 55 /* Guest can handle USOv6 in. */
#define VIRTIO_NET_F_HOST_USO 56 /* Host can handle USO in. */
#define VIRTIO_NET_F_HASH_REPORT 57 /* Supports hash report */
+#define VIRTIO_NET_F_GUEST_HDRLEN 59 /* Guest provides the exact hdr_len
value. */
#define VIRTIO_NET_F_RSS 60 /* Supports RSS RX steering */
#define VIRTIO_NET_F_RSC_EXT 61 /* extended coalescing info */
#define VIRTIO_NET_F_STANDBY 62 /* Act as standby for another device
--
2.39.0