netdev - Re: [patch net-next] net: virtio_net: implement exact header length guest feature

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Date:   Fri, 17 Feb 2023 13:53:55 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, edumazet@...gle.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org
Subject: Re: [patch net-next] net: virtio_net: implement exact header length
 guest feature

Fri, Feb 17, 2023 at 01:22:01PM CET, mst@...hat.com wrote:
>On Fri, Feb 17, 2023 at 01:15:47PM +0100, Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@...dia.com>
>> 
>> virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
>> 
>> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
>> set implicates that the driver provides the exact size of the header.
>> 
>> The driver already complies to fill the correct value. Introduce the
>> feature and advertise it.
>> 
>> Signed-off-by: Jiri Pirko <jiri@...dia.com>
>
>Could you add a bit of motivation just for the record?
>Does this improve performance for some card? By how much?
>Expected to help some future card?

I can get that info, but isn't that rather something to be appended to
the virtio-spec patch? I mean, the feature is there, this is just
implementing it in one driver.


>
>thanks!
>
>
>> ---
>>  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
>