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

From: Alexander Graf <[email-masked]>
Date: Tue, 17 Jun 2014 13:08:36 +0200

On 17.06.14 13:07, Madhavan Srinivasan wrote:
> On Tuesday 17 June 2014 02:24 PM, Alexander Graf wrote:
>> On 14.06.14 23:08, 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 mandates use of
>>> "abs" instruction
>>> (primary opcode 31 and extended opcode 360) as sw breakpoint instruction.
>>> Based on PowerISA v2.01, ABS instruction has been dropped from the
>>> architecture
>>> and treated an illegal instruction.
>>>
>>> Signed-off-by: Madhavan Srinivasan <[email-masked]>
>>> ---
>>>    arch/powerpc/kvm/book3s.c    |  3 ++-
>>>    arch/powerpc/kvm/book3s_hv.c | 23 +++++++++++++++++++----
>>>    2 files changed, 21 insertions(+), 5 deletions(-)
>>>
>>> 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..688421d 100644
>>> --- a/arch/powerpc/kvm/book3s_hv.c
>>> +++ b/arch/powerpc/kvm/book3s_hv.c
>>> @@ -67,6 +67,14 @@
>>>    /* Used as a "null" value for timebase values */
>>>    #define TB_NIL    (~(u64)0)
>>>    +/*
>>> + * SW_BRK_DBG_INT 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 SW_BRK_DBG_INT 0x7c0002d0
>> The instruction we use to trap needs to get exposed to user space via a
>> ONE_REG property.
>>
> Yes. I got to know about that from Bharat (patchset "ppc debug: Add
> debug stub support"). I will change it.
>
>> Also, why don't we use twi always or something else that actually is
>> defined as illegal instruction? I would like to see this shared with
>> book3s_32 PR.
>>
>>> +
>>>    static void kvmppc_end_cede(struct kvm_vcpu *vcpu);
>>>    static int kvmppc_hv_setup_htab_rma(struct kvm_vcpu *vcpu);
>>>    @@ -721,12 +729,19 @@ 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.
>>> +     * To support software breakpoint, we check for the sw breakpoint
>>> +     * instruction to return back to host, if not we just generate a
>>> +     * program interrupt to the guest.
>>>         */
>>>        case BOOK3S_INTERRUPT_H_EMUL_ASSIST:
>>> -        kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
>>> -        r = RESUME_GUEST;
>>> +        if (vcpu->arch.last_inst == SW_BRK_DBG_INT) {
>> Don't access last_inst directly. Instead use the provided helpers.
>>
> Ok. Will look and replace it.
>
>>> +            run->exit_reason = KVM_EXIT_DEBUG;
>>> +            run->debug.arch.address = vcpu->arch.pc;
>>> +            r = RESUME_HOST;
>>> +        } else {
>>> +            kvmppc_core_queue_program(vcpu, 0x80000);
>> magic numbers
> ^^^^^
> I did not understand this?

You're replacing the readable SRR1_PROGILL with the unreadable 0x80000. 
That's bad.

Alex