Well certainly, I know they are different executables. I'm just trying to
understand how the different targets work.

By subsumes, I mean that just looking at the meson.build for i386, you can
see that there are files added to the i386_ss, but not visibly added to the
softmmu target. But the softmmu target has those files built whenever you
configure and build it.

And right that's what I thought. What I'm thinking is that the i386_ss is
compiled with a different implementation for those call backs based on the
target and the user emulation.

On Thu, Jul 15, 2021 at 11:17 AM Peter Maydell <peter.maydell@linaro.org>
wrote:

> On Thu, 15 Jul 2021 at 16:59, Kenneth Adam Miller
> <kennethadammiller@gmail.com> wrote:
> >
> > If I am right, the softmmu/system build target for each architecture
> subsumes the source of the user target.
>
> I'm not sure what you mean by "subsumes" here. Some code in
> QEMU is compiled into both the system and usermode emulators
> (eg most of the CPU emulation code). Some is system mode only
> (eg the device models). Some is usermode only (eg the emulation
> of various system calls).
>
> > What I was wondering is, if the user layer of qemu is used by the
> > user target, and by user layer I mean the using contents of linux-user,
> > then how does the system target receive the user programs instead
> > of the qemu user emulation layer even within a guest that is an
> > entire OS?
>
> qemu-i386 and qemu-system-i386 are different executables.
> They're built differently, and what happens when a usermode
> program running inside a Linux guest inside qemu-system-i386
> executes a syscall instruction is completely different from
> what happens when a program running on qemu-i386 executes that
> instruction. Specifically, there are different versions of the
> x86_cpu_do_interrupt() function: the one for system emulation does
> "update the guest CPU state in the way that the real CPU does when
> an int 0x80 is executed", and the one for usermode emulation does
> "raise a fake exception that will cause execution to return from
> the call to cpu_exec() in linux-user/i386/cpu_loop.c:cpu_loop()
> so it can emulate a syscall".
>
> thanks
> -- PMM
>