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

From: Madhavan Srinivasan <[email-masked]>
Date: Fri, 04 Jul 2014 10:04:54 +0530

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.

>> +
>>   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) ?

Thanks for review
Regards
Maddy

> Alex