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Re: QEMU System and User targets

From: Peter Maydell
Subject: Re: QEMU System and User targets
Date: Thu, 15 Jul 2021 17:17:06 +0100

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".

-- PMM

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