Re: [Xen-devel] Project idea: make QEMU more flexible

[Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>]

On Mon, 2014-01-06 at 12:54 +0000, Wei Liu wrote:
> Hi all
> 
> This idea is to modify QEMU's Makefiles, plus implementing some stubs to
> make it possible to tailor QEMU to a smaller binary.
> 
> The current setup for Xen on X86 is to build i386-softmmu, and uses this
> single binary for two purposes:
> 1. serves as device emulator for HVM guest.
> 2. serves as PV driver backend for PV guest.
> 

Perhaps even KVM would benefit too.
Are you sure we don't emulate any instruction for other purposes?

> Either case CPU emulation is never used because Xen handles that
> already. So we are in fact having a load of unused code in QEMU build.
> 
> What I have in mind is to build a QEMU binary which:
> 1. does not include CPU emulation code at all.
> 2. only includes components that's useful (what's useful is TBD).
> 
> And the rationales behind this idea are:
> 
> 1. Reduce memory footprint. One usecase would be running Xen on embedded
>    platform (X86 or ARM). We would expect the system has very limited
>    resources. The smaller the binary, the better.
> 
> 2. It doesn't make sense to have i386 emulation on ARM platform.
>    Arguably nobody can prevent user from running i386 emulator on ARM
>    platform, but it doesn't make sense in Xen's setup where QEMU is
>    only used as PV device backend on ARM.
> 
> 3. Security concern. It's much easier to audit small code base.
> 
> Please note that I'm not proposing to invalidate all the other usecases.
> I'm only speaking with my Xen developer's hat on, aiming to make QEMU
> more flexible.
> 
> Down to implementation level I only need to (hopefully) add a few stubs
> and create some new CONFIG_* options and move a few things around. It
> might not be as intrusive as one thinks.
> 
> In fact I've already hacked a prototype during Christmas. What's I've
> done so far:
> 
> 1. create target-null which only has some stubs to CPU emulation
>    framework.
> 
> 2. add a few lines to configure / Makefiles*, create
>    default-configs/null-softmmu
> 
> Finally I got a qemu-system-null. And the effect is immediately visible
> -- the size of QEMU binary shrinked from 13MB to 7.6MB. I haven't really
> looked at what device emulation code can be removed so the size can even
> be made smaller.
> 

Well, I would prefer something like a i386-nocpu. The system qemu
emulate is still specific to a given architecture. You can't emulate ARM
with the same executable. So we'd add default-configs/arm-nocpu.mak and
default-config/x86_64-nocpu.mak (i386 don't make much sense anymore).

> What do you think about this idea?
> 
> Thanks
> Wei.
> 

Frediano



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