[Qemu-devel] Re: [PATCH 1/2] Unbreak large mem support by removing kqemu

[Sam Watkins]

I am writing to ask, please can you put kqemu support back in qemu 0.12 as a
build option?  I don't mind if it's not in the default build.  I am happy to
try to improve kqemu if necessary, with some guidance.

Anthony Liguori wrote:
> kqemu introduces a number of restrictions on the i386 target.  The worst is
> that it prevents large memory from working in the default build.

> Furthermore, kqemu is fundamentally flawed in a number of ways.  It relies on
> the TSC as a time source which will not be reliable on a multiple processor
> system in userspace.  Since most modern processors are multicore, this
> severely limits the utility of kqemu.

I suppose anyone with a modern multi-processor x86 will also have VT
extensions, and will be able to run kvm.  These people do not want to use
kqemu, so it doesn't matter that kqemu is broken on a multi-processor system;
no one with a multi-processor system will want to use it.  It's people like me
(with a celeron powered Eee PC, don't laugh!) who can benefit from kqemu.  It
makes qemu usable and useful on these older/smaller systems.  Plain qemu
without kqemu is much slower, it is not really usable.

I think kqemu is useful and it fills a niche for people with less powerful
systems who cannot run kvm, so please put it back into qemu.

> kvm is a viable alternative for people looking to accelerate qemu and has the
> benefit of being supported by the upstream Linux kernel.  If someone can
> implement work arounds to remove the restrictions introduced by kqemu, I'm
> happy to avoid and/or revert this patch.

I am happy to work on this with your guidance, but I don't know whether any
work is actually needed, except to disable kqemu in the default build.

Is there some problem coming from kqemu, which restricts qemu/kvm even in the
case that it is built with --disable-kqemu?  If there is no such problem, you
could revert the patch and simply disable kqemu in the default build.  kqemu is
useful for a lot of people like me who are running slightly older computers.

On this little 1Ghz/1GB Linux Eee PC I am able to run netbsd, openbsd, freebsd,
dragonflybsd, windows XP, Mac OS X, haiku and other systems using kqemu.  I can
even run 4 or 5 of these at the same time.  It works well.  I use this for
testing my software (a programming language), making sure it builds and works
correctly on various systems.


thank-you,

Sam Watkins