Re: [PATCH 2/3] accel/mshv: add gdbstub software breakpoint support

On Mon, Jul 27, 2026 at 05:28:06PM +0300, Doru Blânzeanu wrote:
> Implement software breakpoint based guest debugging for the MSHV
> accelerator, modeled on the WHPX backend. The guest-visible debug
> registers are not used: a breakpoint is an INT1 (opcode 0xf1)
> patched into guest memory, which raises a #DB when executed.
> 
> A partition-wide intercept on #DB is installed the first time a
> breakpoint (or single step) is requested. The intercept is not uninstalled,
> so it stays for the lifetime of the VM and every vmexit mshv_handle_debug()
> decides whether the #DB belongs to gdb (it hit one of our INT1 breakpoints) or
> to the guest; a guest-owned #DB is handed back through a pending exception
> event so the guest takes it through its own IDT.
> Guest INT3 (#BP) is deliberately left to the guest's own IDT and is never
> intercepted.
> 
> Signed-off-by: Doru Blânzeanu <[email protected]>
> ---
>  accel/mshv/mshv-all.c       | 165 +++++++++++++++++++++++++++++++++++-
>  include/system/mshv_int.h   |  14 +++
>  target/i386/mshv/mshv-cpu.c |  86 +++++++++++++++++++
>  3 files changed, 264 insertions(+), 1 deletion(-)
> 
> diff --git a/accel/mshv/mshv-all.c b/accel/mshv/mshv-all.c
> index 72721d0f0d..afcb87b3b8 100644
> --- a/accel/mshv/mshv-all.c
> +++ b/accel/mshv/mshv-all.c
> @@ -29,6 +29,7 @@
>  
>  #include "qemu/accel.h"
>  #include "qemu/guest-random.h"
> +#include "gdbstub/enums.h"
>  #include "accel/accel-ops.h"
>  #include "accel/accel-cpu-ops.h"
>  #include "exec/cpu-common.h"
> @@ -583,6 +584,10 @@ static int mshv_init(AccelState *as, MachineState *ms)
>      s->nr_as = 1;
>      s->as = g_new0(MshvAddressSpace, s->nr_as);
>  
> +    QTAILQ_INIT(&s->sw_breakpoints);
> +
> +    as->gdbstub.sstep_flags = SSTEP_ENABLE;
> +
>      mshv_state = s;
>  
>      mshv_init_irq_routing(s);
> @@ -607,6 +612,153 @@ static int mshv_destroy_vcpu(CPUState *cpu)
>      return 0;
>  }
>  
> +struct MshvSwBreakpoint *mshv_find_sw_breakpoint(CPUState *cpu, vaddr pc)
> +{
> +    struct MshvSwBreakpoint *bp;
> +
> +    QTAILQ_FOREACH(bp, &mshv_state->sw_breakpoints, entry) {
> +        if (bp->pc == pc) {
> +            return bp;
> +        }
> +    }
> +    return NULL;
> +}
> +
> +static int mshv_install_exception_intercept(int vm_fd, uint16_t vector)
> +{
> +    struct hv_input_install_intercept in = {0};
> +    struct mshv_root_hvcall args = {0};
> +    int ret;
> +
> +    in.access_type = 1 << HV_X64_INTERCEPT_ACCESS_TYPE_EXECUTE;
> +    in.intercept_type = HV_INTERCEPT_TYPE_EXCEPTION;
> +    in.intercept_parameter.as_uint64 = vector;
> +
> +    args.code = HVCALL_INSTALL_INTERCEPT;
> +    args.in_sz = sizeof(in);
> +    args.in_ptr = (uint64_t)&in;
> +
> +    ret = mshv_hvcall(vm_fd, &args);
> +    if (ret < 0) {
> +        error_report("Failed to install exception intercept for vector %u",
> +                     vector);
> +    }
> +    return ret;
> +}
> +
> +/*
> + * Install the #DB intercept. This is a permanent, partition-wide latch: it is
> + * installed once on the first debug use and stays for the lifetime of the VM.
> + */
> +static int mshv_init_exception_intercept(void)
> +{
> +    int ret;
> +
> +    if (mshv_state->exception_intercepts_installed) {
> +        return 0;
> +    }
> +
> +    /* Only #DB is needed: gdb breakpoints are INT1 (0xf1) */
> +    ret = mshv_install_exception_intercept(mshv_state->vm, 1); /* #DB */
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    mshv_state->exception_intercepts_installed = true;
> +    return 0;
> +}
> +
> +static int mshv_update_guest_debug(CPUState *cpu)
> +{
> +    if (cpu_single_stepping(cpu)) {
> +        /* Installs exception intercept only on the first call */
> +        return mshv_init_exception_intercept();
> +    }
> +    return 0;
> +}
> +
> +static int mshv_insert_gdbstub_breakpoint(CPUState *cpu, GdbBreakpointType type,
> +                                          vaddr addr, vaddr len)
> +{
> +    struct MshvSwBreakpoint *bp;
> +    int err;
> +
> +    /* We don't use the guest's debug registers. */
> +    if (type != GDB_BREAKPOINT_SW) {
> +        return -ENOSYS;
> +    }
> +
> +    bp = mshv_find_sw_breakpoint(cpu, addr);
> +    if (bp) {
> +        bp->use_count++;
> +        return 0;
> +    }
> +
> +    bp = g_new(struct MshvSwBreakpoint, 1);
> +    bp->pc = addr;
> +    bp->use_count = 1;
> +    err = mshv_arch_insert_sw_breakpoint(cpu, bp);
> +    if (err) {
> +        g_free(bp);
> +        return err;
> +    }
> +
> +    QTAILQ_INSERT_HEAD(&mshv_state->sw_breakpoints, bp, entry);
> +
> +    /* Installs exception intercept only on the first call */
> +    return mshv_init_exception_intercept();
> +}
> +
> +static int mshv_remove_gdbstub_breakpoint(CPUState *cpu, GdbBreakpointType type,
> +                                          vaddr addr, vaddr len)
> +{
> +    struct MshvSwBreakpoint *bp;
> +    int err;
> +
> +    if (type != GDB_BREAKPOINT_SW) {
> +        return -ENOSYS;
> +    }
> +
> +    bp = mshv_find_sw_breakpoint(cpu, addr);
> +    if (!bp) {
> +        return -ENOENT;
> +    }
> +
> +    if (bp->use_count > 1) {
> +        bp->use_count--;
> +        return 0;
> +    }
> +
> +    err = mshv_arch_remove_sw_breakpoint(cpu, bp);
> +    if (err) {
> +        return err;
> +    }
> +
> +    QTAILQ_REMOVE(&mshv_state->sw_breakpoints, bp, entry);
> +    g_free(bp);
> +
> +    return 0;
> +}
> +
> +static void mshv_remove_all_gdbstub_breakpoints(CPUState *cpu)
> +{
> +    struct MshvSwBreakpoint *bp, *next;
> +    CPUState *tmpcpu;
> +
> +    QTAILQ_FOREACH_SAFE(bp, &mshv_state->sw_breakpoints, entry, next) {
> +        if (mshv_arch_remove_sw_breakpoint(cpu, bp) != 0) {
> +            /* Fall back to whichever CPU still has it mapped. */
> +            CPU_FOREACH(tmpcpu) {
> +                if (mshv_arch_remove_sw_breakpoint(tmpcpu, bp) == 0) {
> +                    break;
> +                }
> +            }
> +        }
> +        QTAILQ_REMOVE(&mshv_state->sw_breakpoints, bp, entry);
> +        g_free(bp);
> +    }
> +}
> +
>  static int mshv_cpu_exec(CPUState *cpu)
>  {
>      hv_message mshv_msg;
> @@ -637,6 +789,9 @@ static int mshv_cpu_exec(CPUState *cpu)
>          switch (exit_reason) {
>          case MshvVmExitIgnore:
>              break;
> +        case MshvVmExitDebug:
> +            ret = EXCP_DEBUG;
> +            break;
>          default:
>              ret = EXCP_INTERRUPT;
>              break;
> @@ -708,7 +863,10 @@ static void *mshv_vcpu_thread(void *arg)
>      do {
>          qemu_process_cpu_events(cpu);
>          if (cpu_can_run(cpu)) {
> -            mshv_cpu_exec(cpu);
> +            ret = mshv_cpu_exec(cpu);
> +            if (ret == EXCP_DEBUG) {
> +                cpu_handle_guest_debug(cpu);
> +            }
>          }
>      } while (!cpu->unplug || cpu_can_run(cpu));
>  
> @@ -852,6 +1010,11 @@ static void mshv_accel_ops_class_init(ObjectClass *oc, const void *data)
>      ops->synchronize_pre_loadvm = mshv_cpu_synchronize_pre_loadvm;
>      ops->cpus_are_resettable = mshv_cpus_are_resettable;
>      ops->handle_interrupt = generic_handle_interrupt;
> +
> +    ops->update_guest_debug = mshv_update_guest_debug;
> +    ops->insert_gdbstub_breakpoint = mshv_insert_gdbstub_breakpoint;
> +    ops->remove_gdbstub_breakpoint = mshv_remove_gdbstub_breakpoint;
> +    ops->remove_all_gdbstub_breakpoints = mshv_remove_all_gdbstub_breakpoints;
>  }
>  
>  static const TypeInfo mshv_accel_ops_type = {
> diff --git a/include/system/mshv_int.h b/include/system/mshv_int.h
> index b91c4d661a..9244915eb6 100644
> --- a/include/system/mshv_int.h
> +++ b/include/system/mshv_int.h
> @@ -15,6 +15,7 @@
>  #define QEMU_MSHV_INT_H
>  
>  #include "hw/hyperv/hvhdk.h"
> +#include "exec/vaddr.h"
>  
>  #define MSHV_MSR_ENTRIES_COUNT 64
>  
> @@ -56,6 +57,13 @@ typedef struct MshvAddressSpace {
>      AddressSpace *as;
>  } MshvAddressSpace;
>  
> +struct MshvSwBreakpoint {
> +    vaddr pc;
> +    vaddr saved_insn;
> +    int use_count;
> +    QTAILQ_ENTRY(MshvSwBreakpoint) entry;
> +};
> +
>  struct MshvState {
>      AccelState parent_obj;
>      int vm;
> @@ -70,6 +78,8 @@ struct MshvState {
>      unsigned long *used_gsi_bitmap;
>      unsigned int gsi_count;
>      union hv_partition_processor_features processor_features;
> +    QTAILQ_HEAD(, MshvSwBreakpoint) sw_breakpoints;
> +    bool exception_intercepts_installed;
>  };
>  
>  typedef struct MshvMsiControl {
> @@ -85,6 +95,7 @@ typedef enum MshvVmExit {
>      MshvVmExitIgnore   = 0,
>      MshvVmExitShutdown = 1,
>      MshvVmExitSpecial  = 2,
> +    MshvVmExitDebug    = 3,
>  } MshvVmExit;
>  
>  void mshv_init_mmio_emu(void);
> @@ -98,6 +109,9 @@ int mshv_get_generic_regs(CPUState *cpu, hv_register_assoc *assocs,
>                            size_t n_regs);
>  int mshv_arch_store_vcpu_state(const CPUState *cpu);
>  int mshv_arch_load_vcpu_state(CPUState *cpu);
> +int mshv_arch_insert_sw_breakpoint(CPUState *cpu, struct MshvSwBreakpoint *bp);
> +int mshv_arch_remove_sw_breakpoint(CPUState *cpu, struct MshvSwBreakpoint *bp);
> +struct MshvSwBreakpoint *mshv_find_sw_breakpoint(CPUState *cpu, vaddr pc);
>  void mshv_arch_init_vcpu(CPUState *cpu);
>  void mshv_arch_destroy_vcpu(CPUState *cpu);
>  void mshv_arch_amend_proc_features(
> diff --git a/target/i386/mshv/mshv-cpu.c b/target/i386/mshv/mshv-cpu.c
> index 1c433c408c..2333d3304a 100644
> --- a/target/i386/mshv/mshv-cpu.c
> +++ b/target/i386/mshv/mshv-cpu.c
> @@ -12,6 +12,7 @@
>  
>  #include "qemu/osdep.h"
>  #include "qemu/error-report.h"
> +#include "qemu/main-loop.h"
>  #include "qemu/memalign.h"
>  
>  #include "system/mshv.h"
> @@ -28,6 +29,7 @@
>  #include "emulate/x86_decode.h"
>  #include "emulate/x86_emu.h"
>  #include "emulate/x86_flags.h"
> +#include "gdbstub/enums.h"
>  
>  #include "accel/accel-cpu-target.h"
>  
> @@ -1932,6 +1934,46 @@ static int handle_pio(CPUState *cpu, const struct hyperv_message *msg)
>      return handle_pio_non_str(cpu, &info);
>  }
>  
> +/* Re-inject a guest-owned #DB via a pending event */
> +static int reinject_exception(CPUState *cpu,
> +    struct hv_x64_exception_intercept_message *info)
> +{
> +    hv_register_assoc assoc = { .name = HV_REGISTER_PENDING_EVENT0 };
> +
> +    assoc.value.pending_exception_event.event_pending = 1;
> +    assoc.value.pending_exception_event.event_type =
> +        HV_X64_PENDING_EVENT_EXCEPTION;
> +    assoc.value.pending_exception_event.exception_parameter =
> +        info->exception_parameter;
> +    assoc.value.pending_exception_event.vector = info->exception_vector;
> +
> +    return mshv_set_generic_regs(cpu, &assoc, 1);
> +}
> +
> +/* Check if the intercepted #DB has been set by gdb. */
> +static int handle_debug(CPUState *cpu, hv_message *msg)
> +{
> +    struct hv_x64_exception_intercept_message *info = (void *)msg->payload;
> +
> +    /* Only #DB is intercepted */
> +    if (info->exception_vector != EXCP01_DB) {
> +        return 0;
> +    }
> +
> +    /* INT1 hit: RIP points at the breakpoint */
> +    if (mshv_find_sw_breakpoint(cpu, info->header.rip) != NULL) {
> +        return EXCP_DEBUG;
> +    }
> +
> +    /* Check if single stepping */
> +    if (cpu_single_stepping(cpu)) {
> +        return EXCP_DEBUG;
> +    }
> +
> +    /* The guest's own #DB; caller re-injects it. */
> +    return 0;
> +}
> +
>  int mshv_run_vcpu(int vm_fd, CPUState *cpu, hv_message *msg, MshvVmExit *exit)
>  {
>      int ret;
> @@ -1960,6 +2002,24 @@ int mshv_run_vcpu(int vm_fd, CPUState *cpu, hv_message *msg, MshvVmExit *exit)
>              return MshvVmExitSpecial;
>          }
>          return MshvVmExitIgnore;
> +    case HVMSG_X64_EXCEPTION_INTERCEPT:
maybe we want to move this block into a "handle_exception_intercept()"
function, to keep the switch statement readable.
> +        bql_lock();
> +        if (handle_debug(cpu, msg) == EXCP_DEBUG) {
handle_debug() is a bit unintuitively named, it looks like this is a bool
check (0|EXCP_DEBUG), maybe "bool is_debug_exception()" would be
clearer, and we have can a clear bql_lock/unlock() symmetry w/o early
return arms.
> +            *exit = MshvVmExitDebug;
> +            bql_unlock();
> +            return 0;
> +        }
> +        /* Not ours - hand the #DB back to the guest and keep running. */
> +        ret = reinject_exception(cpu,
> +            (struct hv_x64_exception_intercept_message *)msg->payload);
> +        bql_unlock();
> +        if (ret < 0) {
> +            error_report("failed to reinject exception on vcpu %d",
> +                         cpu->cpu_index);
> +            return -1;
> +        }
> +        *exit = MshvVmExitIgnore;
> +        return 0;
>      default:
>          break;
>      }
> @@ -1973,6 +2033,32 @@ void mshv_remove_vcpu(int vm_fd, int cpu_fd)
>      close(cpu_fd);
>  }
>  
> +int mshv_arch_insert_sw_breakpoint(CPUState *cpu, struct MshvSwBreakpoint *bp)
> +{
> +    static const uint8_t int1 = 0xf1;
> +
> +    if (cpu_memory_rw_debug(cpu, bp->pc, (uint8_t *)&bp->saved_insn, 1, 0) ||
> +        cpu_memory_rw_debug(cpu, bp->pc, (uint8_t *)&int1, 1, 1)) {
> +        return -EINVAL;
> +    }
> +    return 0;
> +}
> +
> +int mshv_arch_remove_sw_breakpoint(CPUState *cpu, struct MshvSwBreakpoint *bp)
> +{
> +    uint8_t int1;
> +
> +    if (cpu_memory_rw_debug(cpu, bp->pc, &int1, 1, 0)) {
> +        return -EINVAL;
> +    }
> +    if (int1 != 0xf1) {
> +        return 0;
> +    }
> +    if (cpu_memory_rw_debug(cpu, bp->pc, (uint8_t *)&bp->saved_insn, 1, 1)) {
> +        return -EINVAL;
> +    }
> +    return 0;
> +}
>  
>  int mshv_create_vcpu(int vm_fd, uint8_t vp_index, int *cpu_fd)
>  {
> -- 
> 2.53.0