Single stepping via GDB/gdbstub 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. Supported scenarios: - Stepping of multiple vcpus; - GDB scheduler locking on and off [2]; - single stepping of kernel code with QEMU while stepping with GDB inside the guest (user space, KGDB). 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 v2 -> v3: - take Alternate Interrupt Location (AIL) into consideration when calculating the Trace Interrupt handler address (this allows single stepping in SLOF code); - check for a new KVM_GUEST_DEBUG_SSTEP capability (still to be submitted to kernel ml); - handle other vcpus (not currently stepping) hitting the single step breakpoint - by ignoring the breakpoint; - handle simultaneous single step by GDB inside guest - by first performing our step into the trace interrupt handler itself and returning to the guest afterwards; - handle single stepping when at the first trace interrupt handler instruction - by displacing the breakpoint to the next instruction; - restore MSR, SRR0, SRR1 after the step, taking into consideration possible mtspr, mtmsr instructions; - use stubs for arch-specific code that will not be implemented by other architectures at this point; https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg04269.html v3 -> v4: - patch 1: fix uninitialized 'offset' variable; - patch 2: squash with patch 7/7 (now 5/5); fix exception vector offset calculation when AIL == 0; - patch 3: squash with 4/7 (now 2/5); - patch 7: introduce ppc_gdb_get_{op,xop,rt,spr} functions; introduce ppc_gdb_read_insn function; define constants for mfmsr, rfid, mtspr extended opcodes; fix bug where instructions would not be properly recognized in SLOF due to guest endianness being different from host's; pass arch_info->address directly into functions that only need the address; improve indentation by returning early when possible. https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg04627.html Fabiano Rosas (5): target/ppc: Move exception vector offset computation into a function kvm-all: Introduce kvm_set_singlestep target/ppc: Move handling of hardware breakpoints to a separate function target/ppc: Refactor kvm_handle_debug target/ppc: support single stepping with KVM HV accel/kvm/kvm-all.c | 16 ++ accel/stubs/kvm-stub.c | 4 + exec.c | 2 +- include/sysemu/kvm.h | 3 + stubs/Makefile.objs | 1 + stubs/kvm-arch-set-singlestep.c | 8 + target/ppc/cpu.h | 16 ++ target/ppc/excp_helper.c | 43 +++-- target/ppc/gdbstub.c | 35 ++++ target/ppc/kvm.c | 312 ++++++++++++++++++++++++++------ 10 files changed, 374 insertions(+), 66 deletions(-) create mode 100644 stubs/kvm-arch-set-singlestep.c -- 2.20.1
