Re: [patch net-next v2] net: virtio_net: implement exact header length guest feature

[Jiri Pirko <jiri@resnulli.us>]

Tue, Feb 21, 2023 at 06:21:16PM CET, mst@redhat.com wrote:
>On Tue, Feb 21, 2023 at 03:47:41PM +0100, Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@nvidia.com>
>> 
>> 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@nvidia.com>
>
>I'm fine with patch itself. with commit log tweak:
>
>Acked-by: Michael S. Tsirkin <mst@redhat.com>

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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