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[213.179.129.39]) by smtp.gmail.com with ESMTPSA id q22-20020a50cc96000000b004acbdf23cd2sm2545400edi.29.2023.02.21.04.32.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Feb 2023 04:32:04 -0800 (PST) Date: Tue, 21 Feb 2023 13:32:03 +0100 From: Jiri Pirko To: "Michael S. Tsirkin" 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: 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> <20230220174110-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230220174110-mutt-send-email-mst@kernel.org> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Mon, Feb 20, 2023 at 11:43:52PM CET, mst@redhat.com wrote: >On Mon, Feb 20, 2023 at 02:56:08PM +0100, Jiri Pirko wrote: >> 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. >> > >I mean if hdr_len is bigger than necessary tap does work. > > >> > >> >> 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. > >OK so do you have hardware which exposes this feature? I believe it is not implemented yet, but it most certainly will be in near future. >That is the bit I am missing. Maybe mention the make >in the commit log so >we know where to turn if we need to make changes here? >Or "under development" if it is not on the market yet. Will put a note in the commit log. > >> >> > >> > >> > >> >> >> >> > >> >> >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 >> >> >> > >> >> > >> > >