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Tsirkin" Cc: netdev@vger.kernel.org, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, edumazet@google.com, virtualization@lists.linux-foundation.org, Vitaly Mireyno References: <20230217121547.3958716-1-jiri@resnulli.us> <20230217072032-mutt-send-email-mst@kernel.org> <20230217083915-mutt-send-email-mst@kernel.org> <20230220074947-mutt-send-email-mst@kernel.org> From: Jason Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 在 2023/2/20 21:56, Jiri Pirko 写道: > Mon, Feb 20, 2023 at 01:55:33PM CET, mst@redhat.com wrote: >> On Mon, Feb 20, 2023 at 09:35:00AM +0100, Jiri Pirko wrote: >>> Fri, Feb 17, 2023 at 02:47:36PM CET, mst@redhat.com wrote: >>>> On Fri, Feb 17, 2023 at 01:53:55PM +0100, Jiri Pirko wrote: >>>>> Fri, Feb 17, 2023 at 01:22:01PM CET, mst@redhat.com wrote: >>>>>> On Fri, Feb 17, 2023 at 01:15:47PM +0100, Jiri Pirko wrote: >>>>>>> From: Jiri Pirko >>>>>>> >>>>>>> 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 >>>>>> 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. >>>> It is more like using it in the driver. It's not like we have to use >>>> everything - it could be useful for e.g. dpdk but not linux. >>>> Implementing it in the Linux driver has support costs - for example what >>>> if there's a bug and sometimes the length is incorrect? >>>> We'll be breaking things. >>> I understand. To my understanding this feature just fixes the original >>> ambiguity in the virtio spec. >>> >>> 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. >>> >>> If you look at the tap implementation, it uses hdr_len to alloc >>> skb linear part. No hint, it counts with the provided value. >>> So if the driver is currently not precise, it breaks tap. >> Well that's only for gso though right? > Yep. > > >> And making it bigger than necessary works fine ... > Well yeah. But tap does not do that, does it? it uses hdr_len directly. tap_get_user() limit the head length: static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,                             struct iov_iter *from, int noblock) {     int good_linear = SKB_MAX_HEAD(TAP_RESERVE); ... > > >>> I will add this to the patch description and send v2. >>> >> I feel this does not answer the question yet, or maybe I am being dense. >> My point was not about making hdr_len precise. My point was that we are >> making a change here for no apparent reason. I am guessing you are not >> doing it for fun - so why? Is there a device with this feature bit >> you are aware of? > Afaik real hw which does emulation of virtio_net would benefit from > that, our hw including. Note that driver can choose to no negotiate this feature, so malicious drivers can still try to use illegal value. Thanks > > >> >> >>>> The patch was submitted by Marvell but they never bothered with >>>> using it in Linux. I guess they are using it for something else? >>>> CC Vitaly who put this in. >>>> >>>>>> 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