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([2400:4050:a840:1e00:78d2:b862:10a7:d486]) by smtp.gmail.com with ESMTPSA id h19-20020aa786d3000000b0068ff267f092sm5777762pfo.216.2023.10.09.01.44.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Oct 2023 01:44:28 -0700 (PDT) Message-ID: <48e20be1-b658-4117-8856-89ff1df6f48f@daynix.com> Date: Mon, 9 Oct 2023 17:44:20 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature To: Willem de Bruijn Cc: Jason Wang , "Michael S. Tsirkin" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-kselftest@vger.kernel.org, bpf@vger.kernel.org, davem@davemloft.net, kuba@kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, kafai@fb.com, songliubraving@fb.com, yhs@fb.com, john.fastabend@gmail.com, kpsingh@kernel.org, rdunlap@infradead.org, willemb@google.com, gustavoars@kernel.org, herbert@gondor.apana.org.au, steffen.klassert@secunet.com, nogikh@google.com, pablo@netfilter.org, decui@microsoft.com, cai@lca.pw, jakub@cloudflare.com, elver@google.com, pabeni@redhat.com, Yuri Benditovich References: <20231008052101.144422-1-akihiko.odaki@daynix.com> <20231008052101.144422-6-akihiko.odaki@daynix.com> Content-Language: en-US From: Akihiko Odaki In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/10/09 17:13, Willem de Bruijn wrote: > On Sun, Oct 8, 2023 at 12:22 AM Akihiko Odaki wrote: >> >> virtio-net have two usage of hashes: one is RSS and another is hash >> reporting. Conventionally the hash calculation was done by the VMM. >> However, computing the hash after the queue was chosen defeats the >> purpose of RSS. >> >> Another approach is to use eBPF steering program. This approach has >> another downside: it cannot report the calculated hash due to the >> restrictive nature of eBPF. >> >> Introduce the code to compute hashes to the kernel in order to overcome >> thse challenges. An alternative solution is to extend the eBPF steering >> program so that it will be able to report to the userspace, but it makes >> little sense to allow to implement different hashing algorithms with >> eBPF since the hash value reported by virtio-net is strictly defined by >> the specification. >> >> The hash value already stored in sk_buff is not used and computed >> independently since it may have been computed in a way not conformant >> with the specification. >> >> Signed-off-by: Akihiko Odaki > >> @@ -2116,31 +2172,49 @@ static ssize_t tun_put_user(struct tun_struct *tun, >> } >> >> if (vnet_hdr_sz) { >> - struct virtio_net_hdr gso; >> + union { >> + struct virtio_net_hdr hdr; >> + struct virtio_net_hdr_v1_hash v1_hash_hdr; >> + } hdr; >> + int ret; >> >> if (iov_iter_count(iter) < vnet_hdr_sz) >> return -EINVAL; >> >> - if (virtio_net_hdr_from_skb(skb, &gso, >> - tun_is_little_endian(tun), true, >> - vlan_hlen)) { >> + if ((READ_ONCE(tun->vnet_hash.flags) & TUN_VNET_HASH_REPORT) && >> + vnet_hdr_sz >= sizeof(hdr.v1_hash_hdr) && >> + skb->tun_vnet_hash) { > > Isn't vnet_hdr_sz guaranteed to be >= hdr.v1_hash_hdr, by virtue of > the set hash ioctl failing otherwise? > > Such checks should be limited to control path where possible There is a potential race since tun->vnet_hash.flags and vnet_hdr_sz are not read at once.