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Subject: Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support
To:     Zha Bin <zhabin@linux.alibaba.com>, linux-kernel@vger.kernel.org
Cc:     virtio-dev@lists.oasis-open.org, slp@redhat.com, mst@redhat.com,
        jing2.liu@linux.intel.com, qemu-devel@nongnu.org,
        chao.p.peng@linux.intel.com, gerry@linux.alibaba.com
References: <cover.1581305609.git.zhabin@linux.alibaba.com>
 <4c3d13be5a391b1fc50416838de57d903cbf8038.1581305609.git.zhabin@linux.alibaba.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <0c71ff9d-1a7f-cfd2-e682-71b181bdeae4@redhat.com>
Date:   Tue, 11 Feb 2020 11:17:38 +0800
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On 2020/2/10 =E4=B8=8B=E5=8D=885:05, 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 specifi=
c
> 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