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[213.179.129.39]) by smtp.gmail.com with ESMTPSA id n4-20020a170906724400b008df97fae83dsm718201ejk.91.2023.02.21.04.29.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Feb 2023 04:29:28 -0800 (PST) Date: Tue, 21 Feb 2023 13:29:27 +0100 From: Jiri Pirko To: Jason Wang Cc: "Michael S. Tsirkin" , netdev@vger.kernel.org, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, edumazet@google.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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Tue, Feb 21, 2023 at 03:38:10AM CET, jasowang@redhat.com wrote: > >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. That's probably why the spec says: 5.1.6.2.2 ... Note: Caution should be taken by the implementation so as to prevent a malicious driver from attacking the device by setting an incorrect hdr_len. And that is exactly what tun does by caping the linear size to SKB_MAX_HEAD(TAP_RESERVE) > >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 >