Subject: Re: [PATCH 1/2 v6] powerpc/kvm: support to handle sw breakpoint

From: Alexander Graf <[email-masked]>
Date: Tue, 09 Sep 2014 23:12:20 +0200

On 09.09.14 19:07, Madhavan Srinivasan wrote:
> This patch adds kernel side support for software breakpoint.
> Design is that, by using an illegal instruction, we trap to hypervisor
> via Emulation Assistance interrupt, where we check for the illegal instruction
> and accordingly we return to Host or Guest. Patch also adds support for
> software breakpoint in PR KVM.
> 
> Signed-off-by: Madhavan Srinivasan <[email-masked]>
> ---
>  arch/powerpc/include/asm/kvm_ppc.h |  6 ++++++
>  arch/powerpc/kvm/book3s.c          |  3 ++-
>  arch/powerpc/kvm/book3s_hv.c       | 41 ++++++++++++++++++++++++++++++++++----
>  arch/powerpc/kvm/book3s_pr.c       |  3 +++
>  arch/powerpc/kvm/emulate.c         | 15 ++++++++++++++
>  5 files changed, 63 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
> index fb86a22..dd83c9a 100644
> --- a/arch/powerpc/include/asm/kvm_ppc.h
> +++ b/arch/powerpc/include/asm/kvm_ppc.h
> @@ -38,6 +38,12 @@
>  #include <asm/paca.h>
>  #endif
>  
> +/*
> + * KVMPPC_INST_SW_BREAKPOINT is debug Instruction
> + * for supporting software breakpoint.
> + */
> +#define KVMPPC_INST_SW_BREAKPOINT	0x00dddd00
> +
>  enum emulation_result {
>  	EMULATE_DONE,         /* no further processing */
>  	EMULATE_DO_MMIO,      /* kvm_run filled with MMIO request */
> diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> index dd03f6b..00e9c9f 100644
> --- a/arch/powerpc/kvm/book3s.c
> +++ b/arch/powerpc/kvm/book3s.c
> @@ -778,7 +778,8 @@ int kvm_arch_vcpu_ioctl_translate(struct kvm_vcpu *vcpu,
>  int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>  					struct kvm_guest_debug *dbg)
>  {
> -	return -EINVAL;
> +	vcpu->guest_debug = dbg->control;
> +	return 0;
>  }
>  
>  void kvmppc_decrementer_func(unsigned long data)
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 27cced9..000fbec 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -725,6 +725,30 @@ static int kvmppc_hcall_impl_hv(unsigned long cmd)
>  	return kvmppc_hcall_impl_hv_realmode(cmd);
>  }
>  
> +static int kvmppc_emulate_debug_inst(struct kvm_run *run,
> +					struct kvm_vcpu *vcpu)
> +{
> +	u32 last_inst;
> +
> +	if (kvmppc_get_last_inst(vcpu, INST_GENERIC, &last_inst) !=
> +					EMULATE_DONE) {
> +		/*
> +		 * Fetch failed, so return to guest and
> +		 * try executing it again.
> +		 */
> +		return RESUME_GUEST;
> +	}
> +
> +	if (last_inst == KVMPPC_INST_SW_BREAKPOINT) {
> +		run->exit_reason = KVM_EXIT_DEBUG;
> +		run->debug.arch.address = kvmppc_get_pc(vcpu);
> +		return RESUME_HOST;
> +	} else {
> +		kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
> +		return RESUME_GUEST;
> +	}
> +}
> +
>  static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  				 struct task_struct *tsk)
>  {
> @@ -807,12 +831,18 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  		break;
>  	/*
>  	 * This occurs if the guest executes an illegal instruction.
> -	 * We just generate a program interrupt to the guest, since
> -	 * we don't emulate any guest instructions at this stage.
> +	 * If the guest debug is disabled, generate a program interrupt
> +	 * to the guest. If guest debug is enabled, we need to check
> +	 * whether the instruction is a software breakpoint instruction.
> +	 * Accordingly return to Guest or Host.
>  	 */
>  	case BOOK3S_INTERRUPT_H_EMUL_ASSIST:
> -		kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
> -		r = RESUME_GUEST;
> +		if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP) {
> +			r = kvmppc_emulate_debug_inst(run, vcpu);
> +		} else {
> +			kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
> +			r = RESUME_GUEST;
> +		}
>  		break;
>  	/*
>  	 * This occurs if the guest (kernel or userspace), does something that
> @@ -922,6 +952,9 @@ static int kvmppc_get_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
>  	long int i;
>  
>  	switch (id) {
> +	case KVM_REG_PPC_DEBUG_INST:
> +		*val = get_reg_val(id, KVMPPC_INST_SW_BREAKPOINT);
> +		break;
>  	case KVM_REG_PPC_HIOR:
>  		*val = get_reg_val(id, 0);
>  		break;
> diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
> index faffb27..6d73708 100644
> --- a/arch/powerpc/kvm/book3s_pr.c
> +++ b/arch/powerpc/kvm/book3s_pr.c

Very nice patch set :). The only thing we're missing now is that Book3S
PR does not allow sw breakpoints to arrive in user mode (MSR.PR == 1),
because there we're never going to arrive at kvmppc_emulate_instruction().

But this can come as a follow-up patch.

Alex