rsinit is a minimalistic single binary init for the initramfs for embedded systems. It's main objective is to mount the root filesystem as fast as possible and then hand over to the actual init program.
The main use-case is, to mount root filesystems that cannot be mounted by the Linux kernel directly. But it can also mount just about anything that the kernel can mount directly based on the command-line.
Currently supported root filesystems are:
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9pfs with USB gadget transport
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filesystems on a dm-verity device
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regular devices or anything that requires just specifying the device and mount options
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nfsroot
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9pfs with virtio transport (for QEMU)