From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received-SPF: Pass (sender SPF authorized) identity=mailfrom; client-ip=209.132.183.28; helo=mx1.redhat.com; envelope-from=lersek@redhat.com; receiver=edk2-devel@lists.01.org Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id 175BF220EE114 for ; Wed, 13 Dec 2017 03:11:48 -0800 (PST) Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 72DF97EA84; Wed, 13 Dec 2017 11:16:27 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-120-40.rdu2.redhat.com [10.10.120.40]) by smtp.corp.redhat.com (Postfix) with ESMTP id 796E777D47; Wed, 13 Dec 2017 11:16:22 +0000 (UTC) To: Paolo Bonzini , Zheng Xiang Cc: edk2-devel@lists.01.org, Ard Biesheuvel , Jordan Justen , Shannon Zhao , Maxime Coquelin References: <20171213031632.11856-1-zhengxiang9@huawei.com> <06533b91-8ff0-1ad2-56fa-5f2c4f56bb19@redhat.com> <7161de6e-99c2-ed81-bbf6-c7a89336c36d@redhat.com> From: Laszlo Ersek Message-ID: Date: Wed, 13 Dec 2017 12:16:21 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: <7161de6e-99c2-ed81-bbf6-c7a89336c36d@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Wed, 13 Dec 2017 11:16:27 +0000 (UTC) Subject: Re: [PATCH] OvmfPkg/VirtioScsiDxe: Allocate all required vrings at VirtioScsiInit X-BeenThere: edk2-devel@lists.01.org X-Mailman-Version: 2.1.22 Precedence: list List-Id: EDK II Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 13 Dec 2017 11:11:48 -0000 Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 12/13/17 10:29, Paolo Bonzini wrote: > On 13/12/2017 09:35, Laszlo Ersek wrote: >> I consider the lack of a "VIRTIO_SCSI_F_MQ" feature bit an issue with >> the virtio specification (and consequently with vhost-scsi), not with >> the guest driver(s). > > VIRTIO_SCSI_F_MQ does not exist because virtio-scsi has _always_ > supported multiqueue and has always had a "num_queues" field in the > configuration space. For virtio-net, VIRTIO_NET_F_MQ does not say > "the device or driver knows about multiqueue", it says "the device or > driver wants to read max_virtqueue_pairs" from configuration space. > It's perfectly fine for a device to propose VIRTIO_NET_F_MQ and set > max_virtqueue_pairs=1, or for a driver to negotiate VIRTIO_NET_F_MQ > and then skip initialization of some virtqueues. > > This also means that Maxime's patch to DPDK is also not enough. :) > Virtio-net actually does have a configuration mechanism for > multiqueue, namely the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command; the > driver sends that command specifying the number of the transmit and > receive queues to use. However, in my understanding, that command is > only needed for the device to configure receive flow steering, so > virtio-scsi doesn't need that either. > >> Perhaps you can update vhost-scsi similarly to the last patch of >> Maxime's v4 series, even without "VIRTIO_SCSI_F_MQ" -- in the >> SET_FEATURES request handler, just destroy the unused virtqueues that >> have not been configured by the guest driver until that time? > > Yes, this is the right solution. We can assume that if the descriptor > address is equal to zero, the queue is not in use. This is not in the > spec as far as I can see, but it is QEMU's assumption. I will send a > patch to the virtio specification. Hmmm, I'm not so sure about this, on a second thought. Reviewing the OVMF code, I see that I added a comment (to all of the virtio drivers actually): > // > // In virtio-1.0, feature negotiation is expected to complete before queue > // discovery, and the device can also reject the selected set of features. > // I added this because of the following sections in the 1.0 spec: - 3.1.1 Driver Requirements: Device Initialization - 3.1.2 Legacy Interface: Device Initialization In particular 3.1.2 writes, "The result was [...] steps 4, 7 and 8 were conflated.". (When I added virtio-1.0 support to OVMF, I paid attention to conform to the new ordering for modern transports, and to keep the ordering unchanged otherwise.) I think this is a problem then; if a 1.0 driver is required to finish feature negotiation (steps 4-6) before configuring the queues (step 7), then the host side cannot derive any clues from the state of the queues when the guest completes step 5 (= set FEATURES_OK). Am I wrong? ... On the other hand, when the driver sets DRIVER_OK (step 8), then the host *can* derive clues from the state of the queues; I think. Thanks Laszlo