Tue, Feb 21, 2023 at 06:21:16PM CET, mst@xxxxxxxxxx wrote:
>On Tue, Feb 21, 2023 at 03:47:41PM +0100, Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@xxxxxxxxxx>
>>
>> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
>> set implicates that the driver provides the exact size of the header.
>
>OK but I feel this is not the important point. The important points are:
>- this bit means device needs this info
>- driver also has to set this bit
>For example one might replace above with:
>
> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
> which when set implicates that device benefits from knowing the exact
> size of the header. For compatiblity, 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.
>
>and the below is ok.
>
>> 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.
>>
>> Signed-off-by: Jiri Pirko <jiri@xxxxxxxxxx>
>
>I'm fine with patch itself. with commit log tweak:
>
>Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Okay. Will do. I will put link to the spec as well
Thanks!
>
>
>> ---
>> 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
>
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