[Qemu-discuss] Qemu: AARCH64: Single Step exception does not work

[Pratyush Anand]

Hi All,

Have anyone tried to test single step exception with ARM64 on Qemu? I 
was testing ARM64 uprobe patches[1] with qemu and I noticed that it does 
not generate single step exception. I also tried kprobe[2], which uses 
single step exception and it does not work. However, these code works 
fine with real silicon.

Test case can be summarized as under:

1. After kernel code is executed, it programs ELR_EL1 with the address 
of instruction which is to be single stepped. Lets say 0x7ffffff004 is 
the address of instruction which is to be single stepped. So, ELR_EL1 
has been programmed with 0x7ffffff004.

2. MDSCR_EL1.SS is set to 1

3. ERET has been called to execute instruction to be single stepped.

With Qemu, I always see
undefined instruction: pc=0000007ffffff008
Code: bad PC value

It seems that Qemu could not notice MDSCR_EL1.SS = 1 and since, kernel 
had written a single valid instruction at location 0x7ffffff004, so it 
raised an undefined exception while executing next invalid instruction.

My Qemu version is:

QEMU emulator version 2.1.2, Copyright (c) 2003-2008 Fabrice Bellard

You may use code in [3] to test single steping.

Please let me know, if any more input is needed to reproduce it.

~Pratyush

[1] https://lkml.org/lkml/2014/12/31/151
[2] https://lkml.org/lkml/2014/11/18/33
[3] https://github.com/pratyushanand/linux.git:ml_arm64_uprobe_devel_v2