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List:       qemu-devel
Subject:    Re: [Qemu-devel] Loadable block drivers?
From:       Stefan Hajnoczi <stefanha () gmail ! com>
Date:       2018-04-04 13:41:22
Message-ID: 20180404134122.GS4467 () stefanha-x1 ! localdomain
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On Tue, Apr 03, 2018 at 11:30:33AM +0800, Fam Zheng wrote:
> On Tue, 04/03 13:17, Lindsay Mathieson wrote:
> > On 3 April 2018 at 13:11, Fam Zheng <famz@redhat.com> wrote:
> > 
> > > On Tue, 04/03 12:59, Lindsay Mathieson wrote:
> > > > Hi all, was looking at developing a block driver for qemu - have examined
> > > > the drivers at:
> > > >
> > > >   https://github.com/qemu/qemu/tree/master/block
> > > >
> > > > And it seems straightforward enough.
> > > >
> > > > One thing that is unclear - all the drivers appear to be compiled
> > > directly
> > > > into qemu. Is there no way to load them dynamically as .so modules?
> > >
> > > './configure --enable-modules' will enable building block drivers as .so
> > > objects, and they are loaded dynamically. These are in-tree .so modules;
> > > out-of-tree modules like in Linux kernel are intentionally forbidden.
> > >
> > > Fam
> > >
> > 
> > 
> > 
> > Rats, I take it that means I can't develop a testing block module and load
> > it with an pre-existing qemu install.
> 
> No, that's not possible.

Depending on what you are trying to do, you could use the blkdebug,
null-co, NBD, or iSCSI drivers to perform your testing.

blkdebug does fault injection (e.g. you can test what happens when
certain I/O requests fail).

null-co is a nop block driver useful for some types of performance
testing and it also supports introducing an artificial delays.

NBD and iSCSI can be used to forward I/O requests to an external server
where you can implement any behavior you want.

We can discuss it more if you can explain what you're trying to do.

Stefan

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