linux-kernel - Re: [RFC PATCH 00/28] Linux Kernel Library

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Date:	Wed, 4 Nov 2015 01:24:03 +0200
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	"Richard W.M. Jones" <rjones@...hat.com>
Cc:	Richard Weinberger <richard.weinberger@...il.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, thehajime@...il.com
Subject: Re: [RFC PATCH 00/28] Linux Kernel Library

On Wed, Nov 4, 2015 at 12:45 AM, Richard W.M. Jones <rjones@...hat.com> wrote:
> On Tue, Nov 03, 2015 at 10:40:29PM +0100, Richard Weinberger wrote:
>> On Tue, Nov 3, 2015 at 9:20 PM, Octavian Purdila
>> <octavian.purdila@...el.com> wrote:
>> > LKL (Linux Kernel Library) is aiming to allow reusing the Linux kernel code
>> > as extensively as possible with minimal effort and reduced maintenance
>> > overhead.
>> >
>> > Examples of how LKL can be used are: creating userspace applications
>> > (running on Linux and other operating systems) that can read or write Linux
>> > filesystems or can use the Linux networking stack, creating kernel drivers
>> > for other operating systems that can read Linux filesystems, bootloaders
>> > support for reading/writing Linux filesystems, etc.
>> >
>> > With LKL, the kernel code is compiled into an object file that can be
>> > directly linked by applications. The API offered by LKL is based on the
>> > Linux system call interface.
>> >
>> > LKL is implemented as an architecture port in arch/lkl. It relies on host
>> > operations defined by the application or a host library (tools/lkl/lib).
>> >
>> > The latest LKL version can be found at git@...hub.com:lkl/linux.git
>>
>> Or more copy&paste friendly: https://github.com/lkl/linux.git
>>
>> > FAQ
>> > ===
>> >
>> > Q: How is LKL different from UML?
>> > A: UML provides a full OS environment (e.g. user/kernel separation, user
>> > processes) and also has requirements (a filesystem, processes, etc.) that
>> > makes it hard to use it for standalone applications. UML also relies
>> > heavily on Linux hosts. On the other hand LKL is designed to be linked
>> > directly with the application and hence does not have user/kernel
>> > separation which makes it easier to use it in standalone applications.
>>
>> So, this is a "liblinux" where applications are directly linked
>> against the kernel.
>> IOW system calls are plain function calls into the kernel?
>>
>> This eliminates UML's most problematic areas, system call handling via ptrace()
>> and virtual memory management via SIGSEGV. :-)
>>
>> > Q: How is LKL different from LibOS?
>> > A: LibOS re-implements high-level kernel APIs for timers, softirqs,
>> > scheduling, sysctl, SLAB/SLUB, etc. LKL behaves like any arch port,
>> > implementing the arch level operations requested by the Linux kernel. LKL
>> > also offers a host interface so that support for multiple hosts can be
>> > easily implemented.
>>
>> Yeah, these re-implementations are what I find most worrisome about LibOS.
>>
>> >
>> > Building LKL the host library and LKL applications
>> > ==================================================
>> >
>> > % cd tools/lkl
>> > % make
>> >
>> > will build LKL as a object file, it will install it in tools/lkl/lib together
>> > with the headers files in tools/lkl/include then will build the host library,
>> > tests and a few of application examples:
>> >
>> > * tests/boot - a simple applications that uses LKL and exercises the basic
>> > LKL APIs
>> >
>> > * fs2tar - a tool that converts a filesystem image to a tar archive
>> >
>> > * cptofs/cpfromfs - a tool that copies files to/from a filesystem image
>>
>> Seeing forward to have a libguestfs port. :-)
>
> Thanks - I was keeping an eye on libos (and on the NetBSD rump kernel
> stuff before), ready to integrate them into libguestfs as soon as they
> offered filesystem access.
>
> It's easy to write a libguestfs-compatible backend, which brings all
> the virt-* tools from libguestfs to the new code.  The UML one looks
> like this:
>
> https://github.com/libguestfs/libguestfs/blob/master/src/launch-uml.c
>

Thanks for the pointers Richard, I am going to take a look at it.

>
> I'm dubious that a lib-based approach could support LVM, partioning,
> ntfs-3g, qcow2, vmdk and all the other libguestfs stuff that relies on
> userspace tools + qemu as well as just the kernel drivers.
> Nevertheless a fast subset of libguestfs supporting just kernel
> filesystem drivers could be useful.
>

LKL uses the full Linux I/O stack and I think LVM and partitioning
should work out of the box. Adding support for qcow2 and vmdk should
be possible as well. ntfs-3g might be problematic.
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