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Re: [Qemu-devel] [RFC] linux-user (mostly syscall.c)

From: J. Mayer
Subject: Re: [Qemu-devel] [RFC] linux-user (mostly syscall.c)
Date: Sat, 03 Nov 2007 13:52:34 +0100

On Sat, 2007-11-03 at 01:21 +0000, Thiemo Seufer wrote:
> Thayne Harbaugh wrote:
> > There are several things that I'd like to see addressed in linux-user.
> > Some of these are to fix bugs, some are to make qemu linux-user more
> > like the Linux kernel, some are to make the internal qemu interfaces
> > more consistent.
> > 
> > An internal coding practice that is being addressed bit-by-bit is that
> > of managing the interface between the host and the target.  Currently
> > this is a bit sloppy and inconsistent (some of which I've contributed
> > to).  There are examples of using target addresses for host pointers and
> > host errnos for target errnos, using different types between target and
> > host that don't sign-extend properly, as well as other things.  This
> > causes compiler warnings to actual run-time bugs.  Currently I'm
> > reviewing all of the linux-user code (mostly syscall.c) to fix these
> > inconsistencies.  I will be writing developer documentation describing
> > the coding practices that should govern the target/host interface and
> > submitting patches for the fixes.
> > 
> > As obvious as it may seem I'll re-state that the linux-user emulation is
> > emulating the Linux kernel (duh!).  There are portions of qemu
> > linux-user that are even excerpted directly from the Linux kernel.
> > Consequently it is useful for internal qemu data and functions to
> > closely mimic the kernel for best code sharing.  There are also
> > advantages to even structuring qemu directly and file organization in
> > similar divisions, groupings and locations.  Some of this organization
> > might lead to good division so that other user/kernel divisions are
> > cleaner (different kernel versions, other OSes - darwin-user and
> > others).
> > 
> > Internal qemu interfaces are consistent - except when they aren't.  This
> > causes coding errors when passing target and host arguments or return
> > codes.  I'll be documenting the coding practices as well as submitting
> > patches to make these consistent.  (That sounds a bit redundant with
> > other things I've mentioned).
> > 
> > I have about 40 patches already worked up that do this.  Some of those
> > patches might be broken up smaller.  The qemu that we've been working
> > with is nearly rock solid (still a few more bugs being wrung out).  It
> > can nearly build an entire Debian arm distribution for an arm target
> > being hosted on x86_64.  We're quite excited to get our patches upstream
> > so that others can benefit and to ease our maintenance overhead.  We're
> > also turning our focus to PPC and other archs.
> > 
> > Please let me know if you support the general idea of the coding changes
> > above: General clean-up, consistent target/host interfaces, file
> > splitting/reorganizing, etc..  In the meantime I'll be putting together
> > the developer documentation/coding guidelines for review.
> FWIW, I agree with everything you said above.

I agree too.
Code cleanup and sanitization is needed there.
I'm just reserved about the code splitting point: as for the vl.h
splitting, it should not lead to get files with only a single or two
small function inside. But it could be great to group the syscalls by
categories, or so. For example, putting all POSIX compliant syscalls in
a single file and using a syscall table could make quite easy to develop
a BSD-user target (I did this in the past, not in Qemu though...). POSIX
compliant interfaces can mostly be shared with Linux ones and a lot of
other syscalls are common to the 3 BSD flavors (Net, Open and Free..).
Being able to add a BSD target sharing the same code would be a proof
the code is flexible and well organized; I guess large parts of the
Darwin user target could also be merged with a FreeBSD user target...

Just my few cents ideas, don't say it has to be implemented soon, just
think keeping those long-term goals in mind may help having a flexible
and clean implementation...

J. Mayer <address@hidden>
Never organized

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