[Qemu-devel] unfaithful emulation of system calls

[John Reiser]

I was surprised that my user-mode applications ran successfully under 
qemu-ppc64 2.10.1
running on Linux on x86_64, but crashed when run directly under Linux on 64-bit 
PowerPC.
Investigation showed that qemu-ppc64 does not emulate system calls faithfully.

On real Linux on PowerPC64, a system call trashes registers {r0, r4-r12, ctr};
qemu-ppc64 preserves them.  [Both preserve: r13-r31, r1 (sp), r2 (TOC), r3 (set 
to
return value), lr (link register).]  Looking at the code in 
qemu/linux-user/syscall.c
(tip commit 92c7ec5cd4d15c76218703f7bd3ca75bd46353b7), I do not see anything 
which
"enforces the ABI", such as by setting all volatile registers to a random value,
or a flag such as 0xA5A5A5...A5A5, or at least to 0.  qemu-user should.

This raises the question, "What *is* the ABI for system calls?".
The documentation  
http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#REG
does not state explicitly that a system call is the same as a subroutine call.
Indeed it isn't, because a system call preserves the link register lr,
but a subroutine call need not.

So, how about qemu-user enforcing the ABI for system calls?

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