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[79.180.62.110]) by smtp.gmail.com with ESMTPSA id q5sm2896866qte.38.2019.09.01.01.26.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 01 Sep 2019 01:26:24 -0700 (PDT) Date: Sun, 1 Sep 2019 04:26:19 -0400 From: "Michael S. Tsirkin" To: Stefano Garzarella Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Stefan Hajnoczi , "David S. Miller" , virtualization@lists.linux-foundation.org, Jason Wang , kvm@vger.kernel.org Subject: Re: [PATCH v4 1/5] vsock/virtio: limit the memory used per-socket Message-ID: <20190901025815-mutt-send-email-mst@kernel.org> References: <20190729153656.zk4q4rob5oi6iq7l@steredhat> <20190729114302-mutt-send-email-mst@kernel.org> <20190729161903.yhaj5rfcvleexkhc@steredhat> <20190729165056.r32uzj6om3o6vfvp@steredhat> <20190729143622-mutt-send-email-mst@kernel.org> <20190730093539.dcksure3vrykir3g@steredhat> <20190730163807-mutt-send-email-mst@kernel.org> <20190801104754.lb3ju5xjfmnxioii@steredhat> <20190801091106-mutt-send-email-mst@kernel.org> <20190801133616.sik5drn6ecesukbb@steredhat> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190801133616.sik5drn6ecesukbb@steredhat> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 01, 2019 at 03:36:16PM +0200, Stefano Garzarella wrote: > On Thu, Aug 01, 2019 at 09:21:15AM -0400, Michael S. Tsirkin wrote: > > On Thu, Aug 01, 2019 at 12:47:54PM +0200, Stefano Garzarella wrote: > > > On Tue, Jul 30, 2019 at 04:42:25PM -0400, Michael S. Tsirkin wrote: > > > > On Tue, Jul 30, 2019 at 11:35:39AM +0200, Stefano Garzarella wrote: > > > > > > (...) > > > > > > > > > > > > > The problem here is the compatibility. Before this series virtio-vsock > > > > > and vhost-vsock modules had the RX buffer size hard-coded > > > > > (VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE = 4K). So, if we send a buffer smaller > > > > > of 4K, there might be issues. > > > > > > > > Shouldn't be if they are following the spec. If not let's fix > > > > the broken parts. > > > > > > > > > > > > > > Maybe it is the time to add add 'features' to virtio-vsock device. > > > > > > > > > > Thanks, > > > > > Stefano > > > > > > > > Why would a remote care about buffer sizes? > > > > > > > > Let's first see what the issues are. If they exist > > > > we can either fix the bugs, or code the bug as a feature in spec. > > > > > > > > > > The vhost_transport '.stream_enqueue' callback > > > [virtio_transport_stream_enqueue()] calls the virtio_transport_send_pkt_info(), > > > passing the user message. This function allocates a new packet, copying > > > the user message, but (before this series) it limits the packet size to > > > the VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE (4K): > > > > > > static int virtio_transport_send_pkt_info(struct vsock_sock *vsk, > > > struct virtio_vsock_pkt_info *info) > > > { > > > ... > > > /* we can send less than pkt_len bytes */ > > > if (pkt_len > VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE) > > > pkt_len = VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE; > > > > > > /* virtio_transport_get_credit might return less than pkt_len credit */ > > > pkt_len = virtio_transport_get_credit(vvs, pkt_len); > > > > > > /* Do not send zero length OP_RW pkt */ > > > if (pkt_len == 0 && info->op == VIRTIO_VSOCK_OP_RW) > > > return pkt_len; > > > ... > > > } > > > > > > then it queues the packet for the TX worker calling .send_pkt() > > > [vhost_transport_send_pkt() in the vhost_transport case] > > > > > > The main function executed by the TX worker is > > > vhost_transport_do_send_pkt() that picks up a buffer from the virtqueue > > > and it tries to copy the packet (up to 4K) on it. If the buffer > > > allocated from the guest will be smaller then 4K, I think here it will > > > be discarded with an error: > > > > > I'm adding more lines to explain better. > > > > static void > > > vhost_transport_do_send_pkt(struct vhost_vsock *vsock, > > > struct vhost_virtqueue *vq) > > > { > ... > > head = vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov), > &out, &in, NULL, NULL); > > ... > > len = iov_length(&vq->iov[out], in); > iov_iter_init(&iov_iter, READ, &vq->iov[out], in, len); > > nbytes = copy_to_iter(&pkt->hdr, sizeof(pkt->hdr), &iov_iter); > if (nbytes != sizeof(pkt->hdr)) { > virtio_transport_free_pkt(pkt); > vq_err(vq, "Faulted on copying pkt hdr\n"); > break; > } > > > > ... > > > nbytes = copy_to_iter(pkt->buf, pkt->len, &iov_iter); > > > > isn't pck len the actual length though? > > > > It is the length of the packet that we are copying in the guest RX > buffers pointed by the iov_iter. The guest allocates an iovec with 2 > buffers, one for the header and one for the payload (4KB). BTW at the moment that forces another kmalloc within virtio core. Maybe vsock needs a flag to skip allocation in this case. Worth benchmarking. See virtqueue_use_indirect which just does total_sg > 1. > > > > if (nbytes != pkt->len) { > > > virtio_transport_free_pkt(pkt); > > > vq_err(vq, "Faulted on copying pkt buf\n"); > > > break; > > > } > > > ... > > > } > > > > > > > > > This series changes this behavior since now we will split the packet in > > > vhost_transport_do_send_pkt() depending on the buffer found in the > > > virtqueue. > > > > > > We didn't change the buffer size in this series, so we still backward > > > compatible, but if we will use buffers smaller than 4K, we should > > > encounter the error described above. So that's an implementation bug then? It made an assumption of a 4K sized buffer? Or even PAGE_SIZE sized buffer? > > > > > > How do you suggest we proceed if we want to change the buffer size? > > > Maybe adding a feature to "support any buffer size"? > > > > > > Thanks, > > > Stefano > > > > > > --