Subject: Re: [PATCH v2] powerpc/kvm: support to handle sw breakpoint

From: Alexander Graf <[email-masked]>
Date: Thu, 03 Jul 2014 13:51:33 +0200

On 01.07.14 10:41, 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.
>
> Patch mandates use of "abs" instruction as sw breakpoint instruction
> (primary opcode 31 and extended opcode 360). Based on PowerISA v2.01,
> ABS instruction has been dropped from the architecture and treated an
> illegal instruction.
>
> 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 |    8 ++++++++
>   arch/powerpc/include/asm/ppc-opcode.h |    5 +++++
>   arch/powerpc/kvm/book3s.c             |    3 ++-
>   arch/powerpc/kvm/book3s_hv.c          |    9 +++++++++
>   arch/powerpc/kvm/book3s_pr.c          |    3 +++
>   arch/powerpc/kvm/emulate.c            |   10 ++++++++++
>   6 files changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
> index f52f656..180d549 100644
> --- a/arch/powerpc/include/asm/kvm_book3s.h
> +++ b/arch/powerpc/include/asm/kvm_book3s.h
> @@ -24,6 +24,14 @@
>   #include <linux/kvm_host.h>
>   #include <asm/kvm_book3s_asm.h>
>   
> +/*
> + * KVMPPC_INST_BOOK3S_DEBUG is debug Instruction for supporting Software Breakpoint.
> + * Instruction mnemonic is ABS, primary opcode is 31 and extended opcode is 360.
> + * Based on PowerISA v2.01, ABS instruction has been dropped from the architecture
> + * and treated an illegal instruction.
> + */
> +#define KVMPPC_INST_BOOK3S_DEBUG	0x7c0002d0

This will still break with LE guests.

> +
>   struct kvmppc_bat {
>   	u64 raw;
>   	u32 bepi;
> diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
> index 3132bb9..3fbb4c1 100644
> --- a/arch/powerpc/include/asm/ppc-opcode.h
> +++ b/arch/powerpc/include/asm/ppc-opcode.h
> @@ -111,6 +111,11 @@
>   #define OP_31_XOP_LHBRX     790
>   #define OP_31_XOP_STHBRX    918
>   
> +/* KVMPPC_INST_BOOK3S_DEBUG -- Software breakpoint Instruction
> + * Instruction mnemonic is ABS, primary opcode is 31 and extended opcode is 360.
> + */
> +#define OP_31_XOP_ABS	    360
> +
>   #define OP_LWZ  32
>   #define OP_LD   58
>   #define OP_LWZU 33
> diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> index c254c27..b40fe5d 100644
> --- a/arch/powerpc/kvm/book3s.c
> +++ b/arch/powerpc/kvm/book3s.c
> @@ -789,7 +789,8 @@ 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 7a12edb..402c1ec 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -725,8 +725,14 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
>   	 * we don't emulate any guest instructions at this stage.
>   	 */
>   	case BOOK3S_INTERRUPT_H_EMUL_ASSIST:
> +	if (kvmppc_get_last_inst(vcpu) == KVMPPC_INST_BOOK3S_DEBUG ) {
> +		run->exit_reason = KVM_EXIT_DEBUG;
> +		run->debug.arch.address = kvmppc_get_pc(vcpu);
> +		r = RESUME_HOST;

Phew - why can't we just go into the normal instruction emulator for 
EMUL_ASSIST?

Alex