KVM: Book3S PR: unbreaking software breakpoints

[Mark Cave-Ayland <mark.cave-ayland@xxxxxxxxxxxx>]

Hi all,

Whilst trying to investigate some issues with MacOS under KVM PR I noticed that when
setting software breakpoints the KVM VCPU would stop as requested, but QEMU's gdbstub
would hang indefinitely.

I eventually traced it down to this code in QEMU's target/ppc/kvm.c:


static int kvm_handle_debug(PowerPCCPU *cpu, struct kvm_run *run)
{
    CPUState *cs = CPU(cpu);
    CPUPPCState *env = &cpu->env;
    struct kvm_debug_exit_arch *arch_info = &run->debug.arch;

    if (cs->singlestep_enabled) {
        return kvm_handle_singlestep();
    }

    if (arch_info->status) {
        return kvm_handle_hw_breakpoint(cs, arch_info);
    }

    if (kvm_find_sw_breakpoint(cs, arch_info->address)) {
        return kvm_handle_sw_breakpoint();
    }


The problem here is that with Book3S PR on my Mac hardware, run->debug.arch.status !=
0 which causes QEMU to think that this is a hardware breakpoint and so the software
breakpoint doesn't get handled correctly.

For comparison both booke.c and e500_emulate.c set debug.arch.status = 0 for software
breakpoints, whereas both book3s_hv.c and book3s_pr.c do not. Given that emulate.c
contains shared code for handling software breakpoints, would the following simple
patch suffice?


diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
index 9f5b8c01c4e1..e77becaad5dd 100644
--- a/arch/powerpc/kvm/emulate.c
+++ b/arch/powerpc/kvm/emulate.c
@@ -282,6 +282,7 @@ int kvmppc_emulate_instruction(struct kvm_run *run, struct
kvm_vcpu *vcpu)
                 */
                if (inst == KVMPPC_INST_SW_BREAKPOINT) {
                        run->exit_reason = KVM_EXIT_DEBUG;
+                       run->debug.arch.status = 0;
                        run->debug.arch.address = kvmppc_get_pc(vcpu);
                        emulated = EMULATE_EXIT_USER;
                        advance = 0;


ATB,

Mark.