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([2400:4050:a840:1e00:78d2:b862:10a7:d486]) by smtp.gmail.com with ESMTPSA id fb3-20020a056a002d8300b0069ee4242f89sm4287361pfb.13.2023.10.09.03.11.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Oct 2023 03:11:29 -0700 (PDT) Message-ID: Date: Mon, 9 Oct 2023 19:11:22 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature Content-Language: en-US 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> <48e20be1-b658-4117-8856-89ff1df6f48f@daynix.com> <6a698c99-6f02-4cfb-a709-ba02296a05f7@daynix.com> 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 19:07, Willem de Bruijn wrote: > On Mon, Oct 9, 2023 at 3:05 AM Akihiko Odaki wrote: >> >> >> >> On 2023/10/09 18:54, Willem de Bruijn wrote: >>> On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki wrote: >>>> >>>> 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. >>> >>> It should not be possible to downgrade the hdr_sz once v1 is selected. >> >> I see nothing that prevents shrinking the header size. >> >> tun->vnet_hash.flags is read after vnet_hdr_sz so the race can happen >> even for the case the header size grows though this can be fixed by >> reordering the two reads. > > One option is to fail any control path that tries to re-negotiate > header size once this hash option is enabled? > > There is no practical reason to allow feature re-negotiation at any > arbitrary time. I think it's a bit awkward interface design since tun allows to reconfigure any of its parameters, but it's certainly possible.