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Subject: Re: [PATCH v1 0/5] dm: dm-user: New target that proxies BIOs to
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To:     Christoph Hellwig <hch@infradead.org>,
        Palmer Dabbelt <palmer@dabbelt.com>
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From:   Bart Van Assche <bvanassche@acm.org>
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Date:   Wed, 23 Dec 2020 08:59:38 -0800
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On 12/22/20 11:48 PM, Christoph Hellwig wrote:
> FYI, a few years ago I spent some time helping a customer to prepare
> their block device in userspace using fuse code for upstreaming, but
> at some point they abandoned the project.  But if for some reason we
> don't want to use nbd I think a driver using the fuse infrastructure
> would be the next logical choice.

Hi Christoph,

Thanks for having shared this information. Since I'm not familiar with the
FUSE code: does this mean translating block device accesses into FUSE_READ
and FUSE_WRITE messages? Does the FUSE kernel code only support exchanging
such messages between kernel and user space via the read() and write()
system calls? I'm asking this since there is already an interface in the
Linux kernel for implementing block devices in user space that uses another
approach, namely a ring buffer for messages and data that is shared between
kernel and user space (documented in Documentation/target/tcmu-design.rst).
Is one system call per read and per write operation fast enough for all
block-device-in-user-space implementations?

Thanks,

Bart.