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[79.176.41.183]) by smtp.gmail.com with ESMTPSA id k50sm3874242qtc.90.2020.02.12.01.16.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Feb 2020 01:16:44 -0800 (PST) Date: Wed, 12 Feb 2020 04:16:39 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: "Liu, Jing2" , Zha Bin , linux-kernel@vger.kernel.org, virtio-dev@lists.oasis-open.org, slp@redhat.com, qemu-devel@nongnu.org, chao.p.peng@linux.intel.com, gerry@linux.alibaba.com Subject: Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support Message-ID: <20200212041554-mutt-send-email-mst@kernel.org> References: <4c3d13be5a391b1fc50416838de57d903cbf8038.1581305609.git.zhabin@linux.alibaba.com> <0c71ff9d-1a7f-cfd2-e682-71b181bdeae4@redhat.com> <5522f205-207b-b012-6631-3cc77dde3bfe@linux.intel.com> <45e22435-08d3-08fe-8843-d8db02fcb8e3@redhat.com> <4c19292f-9d25-a859-3dde-6dd5a03fdf0b@linux.intel.com> <44209f3c-613c-3766-ca83-321b77b0f0dd@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <44209f3c-613c-3766-ca83-321b77b0f0dd@redhat.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 12, 2020 at 05:06:52PM +0800, Jason Wang wrote: > > On 2020/2/12 上午11:54, Liu, Jing2 wrote: > > > > > > On 2/11/2020 3:40 PM, Jason Wang wrote: > > > > > > On 2020/2/11 下午2:02, Liu, Jing2 wrote: > > > > > > > > > > > > On 2/11/2020 12:02 PM, Jason Wang wrote: > > > > > > > > > > On 2020/2/11 上午11:35, Liu, Jing2 wrote: > > > > > > > > > > > > On 2/11/2020 11:17 AM, Jason Wang wrote: > > > > > > > > > > > > > > On 2020/2/10 下午5:05, Zha Bin wrote: > > > > > > > > From: Liu Jiang > > > > > > > > > > > > > > > > Userspace VMMs (e.g. Qemu microvm, Firecracker) > > > > > > > > take advantage of using > > > > > > > > virtio over mmio devices as a lightweight machine model for modern > > > > > > > > cloud. The standard virtio over MMIO transport > > > > > > > > layer only supports one > > > > > > > > legacy interrupt, which is much heavier than > > > > > > > > virtio over PCI transport > > > > > > > > layer using MSI. Legacy interrupt has long work > > > > > > > > path and causes specific > > > > > > > > VMExits in following cases, which would considerably slow down the > > > > > > > > performance: > > > > > > > > > > > > > > > > 1) read interrupt status register > > > > > > > > 2) update interrupt status register > > > > > > > > 3) write IOAPIC EOI register > > > > > > > > > > > > > > > > We proposed to add MSI support for virtio over MMIO via new feature > > > > > > > > bit VIRTIO_F_MMIO_MSI[1] which increases the interrupt performance. > > > > > > > > > > > > > > > > With the VIRTIO_F_MMIO_MSI feature bit > > > > > > > > supported, the virtio-mmio MSI > > > > > > > > uses msi_sharing[1] to indicate the event and vector mapping. > > > > > > > > Bit 1 is 0: device uses non-sharing and fixed > > > > > > > > vector per event mapping. > > > > > > > > Bit 1 is 1: device uses sharing mode and dynamic mapping. > > > > > > > > > > > > > > > > > > > > > I believe dynamic mapping should cover the case of fixed vector? > > > > > > > > > > > > > Actually this bit *aims* for msi sharing or msi non-sharing. > > > > > > > > > > > > It means, when msi sharing bit is 1, device doesn't want > > > > > > vector per queue > > > > > > > > > > > > (it wants msi vector sharing as name) and doesn't want a > > > > > > high interrupt rate. > > > > > > > > > > > > So driver turns to !per_vq_vectors and has to do dynamical mapping. > > > > > > > > > > > > So they are opposite not superset. > > > > > > > > > > > > Thanks! > > > > > > > > > > > > Jing > > > > > > > > > > > > > > > I think you need add more comments on the command. > > > > > > > > > > E.g if I want to map vector 0 to queue 1, how do I need to do? > > > > > > > > > > write(1, queue_sel); > > > > > write(0, vector_sel); > > > > > > > > That's true. Besides, two commands are used for msi sharing mode, > > > > > > > > VIRTIO_MMIO_MSI_CMD_MAP_CONFIG and VIRTIO_MMIO_MSI_CMD_MAP_QUEUE. > > > > > > > > "To set up the event and vector mapping for MSI sharing mode, > > > > driver SHOULD write a valid MsiVecSel followed by > > > > VIRTIO_MMIO_MSI_CMD_MAP_CONFIG/VIRTIO_MMIO_MSI_CMD_MAP_QUEUE > > > > command to map the configuration change/selected queue events > > > > respectively.  " (See spec patch 5/5) > > > > > > > > So if driver detects the msi sharing mode, when it does setup > > > > vq, writes the queue_sel (this already exists in setup vq), > > > > vector sel and then MAP_QUEUE command to do the queue event > > > > mapping. > > > > > > > > > > So actually the per vq msix could be done through this. > > > > Right, per vq msix can also be mapped by the 2 commands if we want. > > > > The current design benefits for those devices requesting per vq msi that > > driver > > > > doesn't have to map during setup each queue, > > > > since we define the relationship by default. > > > > Well since you've defined the dynamic mapping, having some "default" mapping > won't help to reduce the complexity but increase it. > > > > > > > I don't get why you need to introduce MSI_SHARING_MASK which is the > > > charge of driver instead of device. > > > > MSI_SHARING_MASK is just for identifying the msi_sharing bit in > > readl(MsiState) (0x0c4). The device tells whether it wants msi_sharing. > > > > MsiState register: R > > > > le32 { > >     msi_enabled : 1; > >     msi_sharing: 1; > >     reserved : 30; > > }; > > > > The question is why device want such information. > > > > > > > The interrupt rate should have no direct relationship with whether > > > it has been shared or not. > > > > > > > > Btw, you introduce mask/unmask without pending, how to deal with the > > > lost interrupt during the masking then? > > > > > > > > > > For msi non-sharing mode, no special action is needed because we > > > > make the rule of per_vq_vector and fixed relationship. > > > > > > > > Correct me if this is not that clear for spec/code comments. > > > > > > > > > > The ABI is not as straightforward as PCI did. Why not just reuse the > > > PCI layout? > > > > > > E.g having > > > > > > queue_sel > > > queue_msix_vector > > > msix_config > > > > > > for configuring map between msi vector and queues/config > > > > Thanks for the advice. :) > > > > Actually when looking into pci, the queue_msix_vector/msix_config is the > > msi vector index, which is the same as the mmio register MsiVecSel > > (0x0d0). > > > > So we don't introduce two extra registers for mapping even in sharing > > mode. > > > > What do you think? > > > > I'm not sure I get the point, but I still prefer the separate vector_sel > from queue_msix_vector. > > Btw, Michael propose per vq registers which could also work. > > Thanks > Right and I'd even ask a question: do we need shared MSI at all? Is it somehow better than legacy interrupt? And why? Performance numbers please. > > > > > > > > Then > > > > > > vector_sel > > > address > > > data > > > pending > > > mask > > > unmask > > > > > > for configuring msi table? > > > > PCI-like msix table is not introduced to device and instead simply use > > commands to tell the mask/configure/enable. > > > > Thanks! > > > > Jing > > > > > > > > Thanks > > > > > > > > > > Thanks! > > > > > > > > Jing > > > > > > > > > > > > > > > > > > ? > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > > --------------------------------------------------------------------- > > > > > > > > > > > > > > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > > > > > > > For additional commands, e-mail: > > > > > > > virtio-dev-help@lists.oasis-open.org > > > > > > > > > > > > > > > > > > > > >