[v6,2/3] kvm-all: Introduce kvm_set_singlestep

For single stepping (via KVM) of a guest vcpu to work, KVM needs not
only to support the SET_GUEST_DEBUG ioctl but to also recognize the
KVM_GUESTDBG_SINGLESTEP bit in the control field of the
kvm_guest_debug struct.

This patch adds support for querying the single step capability so
that QEMU can decide what to do for the platforms that do not have
such support.

This will allow architecture-specific implementations of a fallback
mechanism for single stepping in cases where KVM does not support it.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
---
 accel/kvm/kvm-all.c         |  9 +++++++++
 accel/stubs/kvm-stub.c      |  5 +++++
 exec.c                      |  2 +-
 include/sysemu/kvm.h        |  3 +++
 stubs/Makefile.objs         |  1 +
 stubs/kvm-arch-singlestep.c | 14 ++++++++++++++
 target/ppc/kvm.c            | 15 +++++++++++++++
 7 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 stubs/kvm-arch-singlestep.c

On Fri, Jan 10, 2020 at 12:13:43PM -0300, Fabiano Rosas wrote:
> For single stepping (via KVM) of a guest vcpu to work, KVM needs not
> only to support the SET_GUEST_DEBUG ioctl but to also recognize the
> KVM_GUESTDBG_SINGLESTEP bit in the control field of the
> kvm_guest_debug struct.
> 
> This patch adds support for querying the single step capability so
> that QEMU can decide what to do for the platforms that do not have
> such support.
> 
> This will allow architecture-specific implementations of a fallback
> mechanism for single stepping in cases where KVM does not support it.
> 
> Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

and ppc parts

Acked-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  accel/kvm/kvm-all.c         |  9 +++++++++
>  accel/stubs/kvm-stub.c      |  5 +++++
>  exec.c                      |  2 +-
>  include/sysemu/kvm.h        |  3 +++
>  stubs/Makefile.objs         |  1 +
>  stubs/kvm-arch-singlestep.c | 14 ++++++++++++++
>  target/ppc/kvm.c            | 15 +++++++++++++++
>  7 files changed, 48 insertions(+), 1 deletion(-)
>  create mode 100644 stubs/kvm-arch-singlestep.c
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index b2f1a5bcb5..a1ea8f9641 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -2668,6 +2668,15 @@ int kvm_update_guest_debug(CPUState *cpu, unsigned long reinject_trap)
>      return data.err;
>  }
>  
> +void kvm_set_singlestep(CPUState *cs, int enabled)
> +{
> +    if (kvm_arch_can_singlestep(cs)) {
> +        kvm_update_guest_debug(cs, 0);
> +    } else {
> +        kvm_arch_emulate_singlestep(cs, enabled);
> +    }
> +}
> +
>  int kvm_insert_breakpoint(CPUState *cpu, target_ulong addr,
>                            target_ulong len, int type)
>  {
> diff --git a/accel/stubs/kvm-stub.c b/accel/stubs/kvm-stub.c
> index 82f118d2df..8773c15713 100644
> --- a/accel/stubs/kvm-stub.c
> +++ b/accel/stubs/kvm-stub.c
> @@ -78,6 +78,11 @@ int kvm_update_guest_debug(CPUState *cpu, unsigned long reinject_trap)
>      return -ENOSYS;
>  }
>  
> +void kvm_set_singlestep(CPUState *cs, int enabled)
> +{
> +    g_assert_not_reached();
> +}
> +
>  int kvm_insert_breakpoint(CPUState *cpu, target_ulong addr,
>                            target_ulong len, int type)
>  {
> diff --git a/exec.c b/exec.c
> index d4b769d0d4..10b61fccb8 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1204,7 +1204,7 @@ void cpu_single_step(CPUState *cpu, int enabled)
>      if (cpu->singlestep_enabled != enabled) {
>          cpu->singlestep_enabled = enabled;
>          if (kvm_enabled()) {
> -            kvm_update_guest_debug(cpu, 0);
> +            kvm_set_singlestep(cpu, enabled);
>          } else {
>              /* must flush all the translated code to avoid inconsistencies */
>              /* XXX: only flush what is necessary */
> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> index 141342de98..43abdb8430 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -247,6 +247,8 @@ bool kvm_memcrypt_enabled(void);
>   */
>  int kvm_memcrypt_encrypt_data(uint8_t *ptr, uint64_t len);
>  
> +bool kvm_arch_can_singlestep(CPUState *cs);
> +void kvm_arch_emulate_singlestep(CPUState *cpu, int enabled);
>  
>  #ifdef NEED_CPU_H
>  #include "cpu.h"
> @@ -259,6 +261,7 @@ int kvm_remove_breakpoint(CPUState *cpu, target_ulong addr,
>                            target_ulong len, int type);
>  void kvm_remove_all_breakpoints(CPUState *cpu);
>  int kvm_update_guest_debug(CPUState *cpu, unsigned long reinject_trap);
> +void kvm_set_singlestep(CPUState *cs, int enabled);
>  
>  int kvm_on_sigbus_vcpu(CPUState *cpu, int code, void *addr);
>  int kvm_on_sigbus(int code, void *addr);
> diff --git a/stubs/Makefile.objs b/stubs/Makefile.objs
> index 8b0ff25508..e6310b878c 100644
> --- a/stubs/Makefile.objs
> +++ b/stubs/Makefile.objs
> @@ -12,6 +12,7 @@ stub-obj-y += get-vm-name.o
>  stub-obj-y += iothread.o
>  stub-obj-y += iothread-lock.o
>  stub-obj-y += is-daemonized.o
> +stub-obj-y += kvm-arch-singlestep.o
>  stub-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
>  stub-obj-y += machine-init-done.o
>  stub-obj-y += migr-blocker.o
> diff --git a/stubs/kvm-arch-singlestep.c b/stubs/kvm-arch-singlestep.c
> new file mode 100644
> index 0000000000..30ee278ba4
> --- /dev/null
> +++ b/stubs/kvm-arch-singlestep.c
> @@ -0,0 +1,14 @@
> +#include "qemu/osdep.h"
> +#include "qemu/error-report.h"
> +#include "sysemu/kvm.h"
> +
> +bool kvm_arch_can_singlestep(CPUState *cs)
> +{
> +    /* for backwards compatibility assume the feature is present */
> +    return true;
> +}
> +
> +void kvm_arch_emulate_singlestep(CPUState *cpu, int enabled)
> +{
> +    warn_report("KVM does not support single stepping");
> +}
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 0bd4a8d399..6fb8687126 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -88,6 +88,7 @@ static int cap_ppc_safe_indirect_branch;
>  static int cap_ppc_count_cache_flush_assist;
>  static int cap_ppc_nested_kvm_hv;
>  static int cap_large_decr;
> +static int cap_ppc_singlestep;
>  
>  static uint32_t debug_inst_opcode;
>  
> @@ -136,6 +137,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>      kvmppc_get_cpu_characteristics(s);
>      cap_ppc_nested_kvm_hv = kvm_vm_check_extension(s, KVM_CAP_PPC_NESTED_HV);
>      cap_large_decr = kvmppc_get_dec_bits();
> +    cap_ppc_singlestep = kvm_vm_check_extension(s, KVM_CAP_PPC_GUEST_DEBUG_SSTEP);
>      /*
>       * Note: setting it to false because there is not such capability
>       * in KVM at this moment.
> @@ -1383,6 +1385,19 @@ static int kvmppc_handle_dcr_write(CPUPPCState *env,
>      return 0;
>  }
>  
> +bool kvm_arch_can_singlestep(CPUState *cs)
> +{
> +    PowerPCCPU *cpu = POWERPC_CPU(cs);
> +    CPUPPCState *env = &cpu->env;
> +
> +    if (cap_ppc_singlestep) {
> +        return true;
> +    }
> +
> +    /* Fallback for when the capability is not available */
> +    return env->excp_model == POWERPC_EXCP_BOOKE || kvmppc_is_pr(kvm_state);
> +}
> +
>  int kvm_arch_insert_sw_breakpoint(CPUState *cs, struct kvm_sw_breakpoint *bp)
>  {
>      /* Mixed endian case is not handled */

On Fri, Jan 17, 2020 at 07:27:48PM +1000, David Gibson wrote:
> On Fri, Jan 10, 2020 at 12:13:43PM -0300, Fabiano Rosas wrote:
> > For single stepping (via KVM) of a guest vcpu to work, KVM needs not
> > only to support the SET_GUEST_DEBUG ioctl but to also recognize the
> > KVM_GUESTDBG_SINGLESTEP bit in the control field of the
> > kvm_guest_debug struct.
> > 
> > This patch adds support for querying the single step capability so
> > that QEMU can decide what to do for the platforms that do not have
> > such support.
> > 
> > This will allow architecture-specific implementations of a fallback
> > mechanism for single stepping in cases where KVM does not support it.
> > 
> > Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> 
> and ppc parts
> 
> Acked-by: David Gibson <david@gibson.dropbear.id.au>

Btw, I'm happy to take this through my tree, but I'd appreciate an ack
from Paolo before touching kvm-all.c and exec.c.