Messages in this thread | | | Subject | Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support | From | Jason Wang <> | Date | Tue, 11 Feb 2020 20:04:24 +0800 |
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On 2020/2/11 下午7:58, Michael S. Tsirkin wrote: > On Tue, Feb 11, 2020 at 03:40:23PM +0800, 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<gerry@linux.alibaba.com> >>>>>>> >>>>>>> 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. I don't get why you >> need to introduce MSI_SHARING_MASK which is the charge of driver instead of >> device. 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? > pending can be an internal device register. as long as device > does not lose interrupts while masked, all's well.
You meant raise the interrupt during unmask automatically?
> > There's value is being able to say "this queue sends no > interrupts do not bother checking used notification area". > so we need way to say that. So I guess an enable interrupts > register might have some value... > But besides that, it's enough to have mask/unmask/address/data > per vq.
Just to check, do you mean "per vector" here?
Thanks
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