Single stepping via GDB/gdbstub in POWER is currently not working with KVM HV. When asking for a single step (stepi), KVM simply ignores the request and execution continues.
This has the direct effect of breaking GDB's 'step', 'stepi', 'next', 'nexti' commands. The 'continue' command is also affected since continuing right after a breakpoint requires that GDB first perform a single step so that the breakpoint can be re-inserted before continuing - in this case the breakpoint is not re-inserted and it won't hit again. The issue here is that single stepping in POWER makes use of an interrupt (Trace Interrupt [1]) that does not reach the hypervisor, so while the single step would happen if properly triggered, it would not cause an exit to KVM so there would be no way of handing control back to GDB. Aside from that, the guest kernel is not prepared to deal with such an interrupt in kernel mode (when not using KGDB, or some other debugging facility) and it causes an Oops. This series implements a "software single step" approach that makes use of: i) the Trace Interrupt to perform the step inside the guest and ii) a breakpoint at the Trace Interrupt handler address to cause a vm exit (Emulation Assist) so that we can return control to QEMU. With (i), we basically get the single step for free, without having to discover what are the possible targets of instructions that divert execution. With (ii), we hide the single step from the guest and keep all of the step logic in QEMU. This was so far tested with single and multiple vcpus and with GDB scheduler locking on and off [2]. I have not fully explored yet the potential issues when using debuggers simultaneously inside and outside the guest, however I was able to single step the ptrace code while single stepping a userspace program inside the guest with GDB. I'm looking for feedback on the general approach before I develop this further. 1- PowerISA Section 6.5.15 - Trace Interrupt 2- https://sourceware.org/gdb/onlinedocs/gdb/All_002dStop-Mode.html v1 -> v2: - split in more patches to facilitate review - use extract32 for decoding instruction instead of open-coding - add more people to CC https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg03738.html Fabiano Rosas (3): target/ppc: Add macro definitions for relocated interrupt vectors offsets kvm-all: Introduce kvm_set_singlestep target/ppc: support single stepping with KVM HV accel/kvm/kvm-all.c | 10 +++++++ exec.c | 1 + include/sysemu/kvm.h | 4 +++ target/arm/kvm.c | 4 +++ target/i386/kvm.c | 4 +++ target/mips/kvm.c | 4 +++ target/ppc/cpu.h | 3 ++ target/ppc/excp_helper.c | 4 +-- target/ppc/kvm.c | 65 +++++++++++++++++++++++++++++++++++++++- target/s390x/kvm.c | 4 +++ 10 files changed, 100 insertions(+), 3 deletions(-) -- 2.17.1
