Re: Building QEMU as a shared library

[Stefan Hajnoczi]

On Wed, Dec 15, 2021 at 08:18:53AM +0000, Amir Gonnen wrote:
> Before sending a patch, I would like to check if it's of interest to the 
> community.
> 
> My goal is to simulate a mixed architecture system.
> Today QEMU strongly assumes that the simulated system is a *single 
> architecture*.
> Changing this assumption and supporting mixed architecture in QEMU proved to 
> be
> non-trivial and may require significant development effort. Common code such 
> as
> TCG and others explicitly include architecture specific header files, for 
> example.

Hi Amir,
Simulating heterogenous machines comes up from periodically. So far no
one has upstreamed a solution but there is definitely interest.

I suggest going ahead and posting the code even if it's not cleaned up.

> A possible solution, discussed on https://stackoverflow.com/q/63229262/619493 
> is to
> separate the simulation to multiple processes (as done by Xilinx) and to use 
> some form
> of Interprocess Communication channel between them.
> Such solution has several disadvantages:
> 
> - Harder to synchronize simulation between processes
> - Performance impact of Interprocess Communication
> - Harder to debug, profile and maintain
> 
> Instead, I would like to suggest a new approach we use at Neuroblade to 
> achieve this:
> Build QEMU as a shared library that can be loaded and used directly in a 
> larger simulation.
> Today we build qemu-system-nios2 shared library and load it from 
> qemu-system-x86_64 in order
> to simulate an x86_64 system that also consists of multiple nios2 cores.
> In our simulation, two independent "main" functions are running on different 
> threads, and
> simulation synchronization is reduced to synchronizing threads.
> 
> To achieve this, I needed to do the following changes in QEMU:
> 
> 1. Avoid Glib global context. Use a different context (g_main_context_new) 
> for each QEMU instance.
> 2. Change meson.build to build QEMU as a shared library (with PIC enabled for 
> static libraries)
> 3. Define a C API for the library and export it (with a -Wl,--version-script)
> 
> These changes seem enough for simulating mixed architecture system on a 
> single process.
> 
> If this approach sounds useful, I'll be happy to send patches.
> I'd appreciate if you could provide your feedback!

I'm curious how much synchronization and IPC there is between the QEMU
shared libraries? I would have guessed that the pain of making
communication work efficiently between processes would be less than the
pain of solving global state bugs related to shared libraries within a
single process.

Were there issues with POSIX signal handlers?

Stefan

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