On Wed, Feb 22, 2023 at 09:06:38AM +0100, Jiri Pirko wrote: > From: Jiri Pirko <jiri@xxxxxxxxxx> > > Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when > which when which when which when is probably unintentional :) >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@xxxxxxxxxx> > Reviewed-by: Parav Pandit <parav@xxxxxxxxxx> > Reviewed-by: Alvaro Karsz <alvaro.karsz@xxxxxxxxxxxxx> > Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > --- > 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
