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Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt featu

From: Liu, Jing2
Subject: Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support
Date: Tue, 11 Feb 2020 14:02:01 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1

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<address@hidden>

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

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.



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,


"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.

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.






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