Subject: Re: [PATCH v4] powerpc/kvm: support to handle sw breakpoint

From: Alexander Graf <[email-masked]>
Date: Thu, 21 Aug 2014 11:10:21 +0200

On 20.08.14 07:52, 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.
> 
> Changes v3->v4:
>  Made changes to code comments and removed #define of zero opcode
>  Added a new function to handle the debug instruction emulation in book3s_hv
>  Rebased the code to latest upstream source.
> 
> Changes v2->v3:
>  Changed the debug instructions. Using the all zero opcode in the instruction word
>   as illegal instruction as mentioned in Power ISA instead of ABS
>  Removed reg updated in emulation assist and added a call to
>   kvmppc_emulate_instruction for reg update.
> 
> Changes v1->v2:
> 
>  Moved the debug instruction #def to kvm_book3s.h. This way PR_KVM can also share it.
>  Added code to use KVM get one reg infrastructure to get debug opcode.
>  Updated emulate.c to include emulation of debug instruction incase of PR_KVM.
>  Made changes to commit message.
> 
> Signed-off-by: Madhavan Srinivasan <[email-masked]>
> ---
>  arch/powerpc/include/asm/kvm_book3s.h |  7 +++++++
>  arch/powerpc/kvm/book3s.c             |  3 ++-
>  arch/powerpc/kvm/book3s_hv.c          | 32 ++++++++++++++++++++++++++++++--
>  arch/powerpc/kvm/book3s_pr.c          |  3 +++
>  arch/powerpc/kvm/emulate.c            | 11 +++++++++++
>  5 files changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
> index 6acf0c2..a1944f8 100644
> --- a/arch/powerpc/include/asm/kvm_book3s.h
> +++ b/arch/powerpc/include/asm/kvm_book3s.h
> @@ -24,6 +24,13 @@
>  #include <linux/kvm_host.h>
>  #include <asm/kvm_book3s_asm.h>
>  
> +/*
> + * KVMPPC_INST_BOOK3S_DEBUG is debug Instruction for supporting Software Breakpoint.
> + * Based on PowerISA v2.07, Instruction with primary opcode 0 will be treated as illegal
> + * instruction.
> + */
> +#define KVMPPC_INST_BOOK3S_DEBUG	0x00dddd00

Please change the BookE version of this as well, put the define in a
common header and use a non book specific name.

> +
>  struct kvmppc_bat {
>  	u64 raw;
>  	u32 bepi;
> 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..0a92e45 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -725,6 +725,14 @@ static int kvmppc_hcall_impl_hv(unsigned long cmd)
>  	return kvmppc_hcall_impl_hv_realmode(cmd);
>  }
>  
> +static int kvmppc_emulate_debug_instruction_hv(struct kvm_run *run,
> +					struct kvm_vcpu *vcpu)
> +{
> +	run->exit_reason = KVM_EXIT_DEBUG;
> +	run->debug.arch.address = kvmppc_get_pc(vcpu);
> +	return 0;
> +}
> +
>  static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  				 struct task_struct *tsk)
>  {
> @@ -811,9 +819,26 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  	 * we don't emulate any guest instructions at this stage.

This comment is no longer true, it should get changed.

>  	 */
>  	case BOOK3S_INTERRUPT_H_EMUL_ASSIST:
> -		kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
> -		r = RESUME_GUEST;
> +	{
> +		u32 last_inst;
> +		if(kvmppc_get_last_inst(vcpu, INST_GENERIC, &last_inst) !=
> +					EMULATE_DONE) {

Please only do this if debugging is active.

> +			/*
> +			 * Fetch failed, so return to guest and
> +			 * try executing it again.
> +			 */
> +			r = RESUME_GUEST;
> +		} else {
> +			if (last_inst == KVMPPC_INST_BOOK3S_DEBUG) {
> +				kvmppc_emulate_debug_instruction_hv(run, vcpu);
> +				r = RESUME_HOST;
> +			} else {
> +				kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
> +				r = RESUME_GUEST;
> +			}
> +		}
>  		break;
> +	}
>  	/*
>  	 * This occurs if the guest (kernel or userspace), does something that
>  	 * is prohibited by HFSCR.  We just generate a program interrupt to
> @@ -922,6 +947,9 @@ 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_BOOK3S_DEBUG);
> +		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..cd2a39d 100644
> --- a/arch/powerpc/kvm/book3s_pr.c
> +++ b/arch/powerpc/kvm/book3s_pr.c
> @@ -1319,6 +1319,9 @@ static int kvmppc_get_one_reg_pr(struct kvm_vcpu *vcpu, u64 id,
>  	int r = 0;
>  
>  	switch (id) {
> +	case KVM_REG_PPC_DEBUG_INST:
> +		*val = get_reg_val(id, KVMPPC_INST_BOOK3S_DEBUG);
> +		break;
>  	case KVM_REG_PPC_HIOR:
>  		*val = get_reg_val(id, to_book3s(vcpu)->hior);
>  		break;
> diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
> index e96b50d..68f38b2 100644
> --- a/arch/powerpc/kvm/emulate.c
> +++ b/arch/powerpc/kvm/emulate.c
> @@ -274,6 +274,17 @@ int kvmppc_emulate_instruction(struct kvm_run *run, struct kvm_vcpu *vcpu)
>  		}
>  		break;
>  
> +	case 0:
> +		/*
> +		 * Instruction with primary opcode 0. Based on PowerISA
> +		 * these are illegal instructions.
> +		 */
> +		run->exit_reason = KVM_EXIT_DEBUG;

This is wrong. We need to check that the instruction is 0x00dddd00. Any
other op0 instruction should trigger an invalid instruction trap.

Alex

> +		run->debug.arch.address = kvmppc_get_pc(vcpu);
> +		emulated = EMULATE_EXIT_USER;
> +		advance = 0;
> +		break;
> +
>  	default:
>  		emulated = EMULATE_FAIL;
>  	}
>