Re: [PATCH 2/2] KVM: PPC: booke/bookehv: Add guest debug support

[Scott Wood <scottwood@freescale.com>]

On 08/16/2012 03:48 AM, Bhushan Bharat-R65777 wrote:
>>>>> diff --git a/arch/powerpc/include/asm/kvm.h
>>>>> b/arch/powerpc/include/asm/kvm.h index 3c14202..da71c84 100644
>>>>> --- a/arch/powerpc/include/asm/kvm.h
>>>>> +++ b/arch/powerpc/include/asm/kvm.h
>>>>> @@ -25,6 +25,7 @@
>>>>>  /* Select powerpc specific features in <linux/kvm.h> */  #define
>>>>> __KVM_HAVE_SPAPR_TCE  #define __KVM_HAVE_PPC_SMT
>>>>> +#define __KVM_HAVE_GUEST_DEBUG
>>>>>
>>>>>  struct kvm_regs {
>>>>>  	__u64 pc;
>>>>> @@ -265,10 +266,19 @@ struct kvm_fpu {  };
>>>>>
>>>>>  struct kvm_debug_exit_arch {
>>>>> +	__u32 exception;
>>>>> +	__u32 pc;
>>>>> +	__u32 status;
>>>>>  };
>>>>
>>>> PC must be 64-bit.  What goes in "status" and "exception"?
> 
> status ->  exit because of h/w breakpoint, watchpoint (read, write or
> both) and software breakpoint.
>
> exception -> returns the exception number. If the exit is not handled
> (say not h/w breakpoint or software breakpoint set for this address)
> by qemu then it is supposed to inject the exception to guest. This is
> how it is implemented for x86.

Where is this documented (including the specific values that are possible)?

>>>>> +#define KVM_GUESTDBG_USE_SW_BP          0x00010000
>>>>> +#define KVM_GUESTDBG_USE_HW_BP          0x00020000
>>>>
>>>> Where do these get used?  Any reason for these particular values?  If
>>>> you're trying to create a partition where the upper half is generic
>>>> and the lower half is arch-specific, say so.
>>>
>>> KVM_SET_GUEST_DEBUG ioctl used to set/unset debug interrupts, which
>>> have a "u32 control" element. We have inherited this mechanism from
>>> x86 implementation and it looks like lower 16 bits are generic (like
>>> KVM_GUESTDBG_ENBLE, KVM_GUESTDBG_SINGLESTEP etc and upper 16 bits are
>>> Architecture specific.
>>>
>>> I will add a comment to describe this.
>>
>> I don't think the sw/hw distinction belongs here -- it should be per breakpoint.
> 
> KVM does not track the software breakpoint, so it is not per breakpoint.
> In KVM, when KVM_GUESTDBG_USE_SW_BP flag is set and special trap instruction is executed by guest then exit to userspace.

Can both types of breakpoint be set at the same time?

>>>>> +		run->exit_reason = KVM_EXIT_DEBUG;
>>>>> +		run->debug.arch.pc = vcpu->arch.pc;
>>>>> +		run->debug.arch.exception = exit_nr;
>>>>> +		run->debug.arch.status = 0;
>>>>> +		kvmppc_account_exit(vcpu, DEBUG_EXITS);
>>>>> +		return RESUME_HOST;
>>>>
>>>> The interface isn't (clearly labelled as) booke specific, but you
>>>> return booke- specific exception numbers.  How's userspace supposed
>>>> to know what to do with them?  What do you plan on doing with them in QEMU?
>>>
>>> This is booke specific.
>>
>> Then put booke in the name,
> 
> Which data structure name should have booke?

Anything that's booke specific.

-Scott