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

From: Madhavan Srinivasan <[email-masked]>
Date: Fri, 04 Jul 2014 13:02:28 +0530

On Friday 04 July 2014 12:18 PM, Alexander Graf wrote:
> 
> On 04.07.14 06:34, Madhavan Srinivasan wrote:
>> On Thursday 03 July 2014 05:21 PM, Alexander Graf wrote:
>>> 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.
>>>
>> I am told to try with all 0s opcode. So rewriting the patch.
> 
> The problem with "all 0s" is that it's reasonably likely to occur on
> real world code. Hence Segher was proposing something like 0x00dddd00
> which should be the same regardless of endianness, but has a certain
> appeal of intentional placement ;).
> 

Ok Sure.

>>
>>>> +
>>>>    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?
>>>
>> IIUC, using the emulation_assist_interrupt function (kernel/trap.c) ?
> 
> I was more thinking of kvmppc_emulate_instruction() :).
> 

This makes sense. Can use the same call for pr kvm also. awesome :)

> Alex