From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 21 Feb 2023 18:18:01 -0500 From: "Michael S. Tsirkin" Subject: Re: [PATCH v9] virtio-net: support inner header hash Message-ID: <20230221180945-mutt-send-email-mst@kernel.org> References: <20230218143715.841-1-hengqi@linux.alibaba.com> <20230221115147-mutt-send-email-mst@kernel.org> <20230221161551-mutt-send-email-mst@kernel.org> <20230221163930-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=us-ascii Content-Disposition: inline To: Parav Pandit Cc: Heng Qi , "virtio-comment@lists.oasis-open.org" , "virtio-dev@lists.oasis-open.org" , Jason Wang , Yuri Benditovich , Cornelia Huck , Xuan Zhuo List-ID: On Tue, Feb 21, 2023 at 10:32:11PM +0000, Parav Pandit wrote: > > > From: Michael S. Tsirkin > > Sent: Tuesday, February 21, 2023 4:46 PM > > > > What is this information driver can't observe? It sees all the packets after all, > > we are not stripping tunneling headers. > Just the tunnel type. > If/when that tunnel header is stripped, it gets complicated where tunnel type is still present in the virtio_net_hdr because hash_report_tunnel feature bit is negotiated. whoever strips off the tunnel has I imagine strip off the virtio net hdr too - everything else in it such as gso type refers to the outer packet. > > I also don't really know what are upper layer drivers - for sure layering of > > drivers is not covered in the spec for now so I am not sure what do you mean by > > that. The risk I mentioned is leaking the information *on the network*. > > > Got it. > > > > > > > > > > > > > \begin{lstlisting} struct virtio_net_rss_config { > > > > > > > > le32 hash_types; > > > > > > > > + le32 hash_tunnel_types; > > > > > > > This field is not needed as device config space advertisement > > > > > > > for the support > > > > > > is enough. > > > > > > > > > > > > > > If the intent is to enable hashing for the specific tunnel(s), > > > > > > > an individual > > > > > > command is better. > > > > > > > > > > > > new command? I am not sure why we want that. why not handle > > > > > > tunnels like we do other protocols? > > > > > > > > > > I didn't follow. > > > > > We probably discussed in another thread that to set M bits, it is > > > > > wise to avoid > > > > setting N other bits just to keep the command happy, where N >>> M > > > > and these N have a very strong relation in hw resource setup and packet > > steering. > > > > > Any examples of 'other protocols'? > > > > > > > > #define VIRTIO_NET_HASH_TYPE_IPv4 (1 << 0) > > > > #define VIRTIO_NET_HASH_TYPE_TCPv4 (1 << 1) > > > > #define VIRTIO_NET_HASH_TYPE_UDPv4 (1 << 2) > > > > > > > > this kind of thing. > > > > > > > > I don't see how a tunnel is different fundamentally. Why does it > > > > need its own field? > > > > > > Driver is in control to enable/disable tunnel based inner hash acceleration > > only when its needed. > > > This way certain data path hw parsers can be enabled/disabled. > > > Without this it will be always enabled even if there may not be any user of it. > > > Device has scope to optimize this flow. > > > > I feel you misunderstand the question. Or maybe I misunderstand what you are > > proposing. So tunnels need their own bits. But why a separate field and not just > > more bits along the existing ones? > > Because the hashing is not covering the outer header contents. > > We may be still not discussing the same. > So let me refresh the context. > > The question of discussion was, > Scenario: > 1. device advertises the ability to hash on the inner packet header. > 2. device prefers that driver enable it only when it needs to use this extra packet parser in hardware. > > There are 3 options. > a. Because the feature is negotiated, it means it is enabled for all the tunnel types. > Pros: > 1. No need to extend cvq cmd. > Cons: > 1. device parser is always enabled, and the driver never uses it. This may result in inferior rx performance. > > b. Since the feature is useful in a narrow case of sw-based vxlan etc driver, better not to enable hw for it. > Hence, have the knob to explicitly enable in hw. > So have the cvq command. > b.1 should it be combined with the existing command? > Cons: > a. when the driver wants to enable hash on inner, it needs to supply the exact same RSS config as before. Sw overhead with no gain. > b. device needs to parse new command value, compare with old config, and drop the RSS config, just enable inner hashing hw parser. > Or destroy the old rss config and re-apply. This results in weird behavior for the short interval with no apparent gain. > > b.2 should it be on its own command? > Pros: > a. device and driver doesn't need to bother about b.1.a and b.1.b. > b. still benefits from not always enabling hw parser, as this is not a common case. > c. has the ability to enable when needed. I prefer b.1. With reporting of the tunnel type gone I don't see a fundamental difference between hashing over tunneling types and other protocol types we support. It's just a flag telling device over which bits to calculate the hash. We don't have a separate command for hashing of TCPv6, why have it for vxlan? Extending with more HASH_TYPE makes total sense to me, seems to fit better with the existing design and will make patch smaller. -- MST