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Tsirkin" To: Jiri Pirko Cc: netdev@vger.kernel.org, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, edumazet@google.com, jasowang@redhat.com, virtualization@lists.linux-foundation.org, Vitaly Mireyno Subject: Re: [patch net-next] net: virtio_net: implement exact header length guest feature Message-ID: <20230220074947-mutt-send-email-mst@kernel.org> References: <20230217121547.3958716-1-jiri@resnulli.us> <20230217072032-mutt-send-email-mst@kernel.org> <20230217083915-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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? And making it bigger than necessary works fine ... > 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? > > > > >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 > >> > > >