linux-kernel - Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support

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Date:   Tue, 11 Feb 2020 11:17:38 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Zha Bin <zhabin@...ux.alibaba.com>, linux-kernel@...r.kernel.org
Cc:     virtio-dev@...ts.oasis-open.org, slp@...hat.com, mst@...hat.com,
        jing2.liu@...ux.intel.com, qemu-devel@...gnu.org,
        chao.p.peng@...ux.intel.com, gerry@...ux.alibaba.com
Subject: Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support


On 2020/2/10 下午5:05, Zha Bin wrote:
> From: Liu Jiang<gerry@...ux.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?

Thanks