Re: [PATCH 2/2] kvm/arm64: Detach ESR operator from vCPU struct

[Gavin Shan <gshan@redhat.com>]

Hi Mark,

On 6/29/20 9:00 PM, Mark Rutland wrote:
> On Mon, Jun 29, 2020 at 07:18:41PM +1000, Gavin Shan wrote:
>> There are a set of inline functions defined in kvm_emulate.h. Those
>> functions reads ESR from vCPU fault information struct and then operate
>> on it. So it's tied with vCPU fault information and vCPU struct. It
>> limits their usage scope.
>>
>> This detaches these functions from the vCPU struct by introducing an
>> other set of inline functions in esr.h to manupulate the specified
>> ESR value. With it, the inline functions defined in kvm_emulate.h
>> can call these inline functions (in esr.h) instead. This shouldn't
>> cause any functional changes.
>>
>> Signed-off-by: Gavin Shan <gshan@redhat.com>
> 
> TBH, I'm not sure that this patch makes much sense on its own.
> 
> We already use vcpu_get_esr(), which is the bit that'd have to change if
> we didn't pass the vcpu around, and the new helpers are just consuming
> the value in a sifferent way rather than a necessarily simpler way.
> 
> Further comments on that front below.
> 
>> ---
>>   arch/arm64/include/asm/esr.h         | 32 +++++++++++++++++++++
>>   arch/arm64/include/asm/kvm_emulate.h | 43 ++++++++++++----------------
>>   2 files changed, 51 insertions(+), 24 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
>> index 035003acfa87..950204c5fbe1 100644
>> --- a/arch/arm64/include/asm/esr.h
>> +++ b/arch/arm64/include/asm/esr.h
>> @@ -326,6 +326,38 @@ static inline bool esr_is_data_abort(u32 esr)
>>   	return ec == ESR_ELx_EC_DABT_LOW || ec == ESR_ELx_EC_DABT_CUR;
>>   }
>>   
>> +#define ESR_DECLARE_CHECK_FUNC(name, field)	\
>> +static inline bool esr_is_##name(u32 esr)	\
>> +{						\
>> +	return !!(esr & (field));		\
>> +}
>> +#define ESR_DECLARE_GET_FUNC(name, mask, shift)	\
>> +static inline u32 esr_get_##name(u32 esr)	\
>> +{						\
>> +	return ((esr & (mask)) >> (shift));	\
>> +}
>> +
>> +ESR_DECLARE_CHECK_FUNC(il_32bit,   ESR_ELx_IL);
>> +ESR_DECLARE_CHECK_FUNC(condition,  ESR_ELx_CV);
>> +ESR_DECLARE_CHECK_FUNC(dabt_valid, ESR_ELx_ISV);
>> +ESR_DECLARE_CHECK_FUNC(dabt_sse,   ESR_ELx_SSE);
>> +ESR_DECLARE_CHECK_FUNC(dabt_sf,    ESR_ELx_SF);
>> +ESR_DECLARE_CHECK_FUNC(dabt_s1ptw, ESR_ELx_S1PTW);
>> +ESR_DECLARE_CHECK_FUNC(dabt_write, ESR_ELx_WNR);
>> +ESR_DECLARE_CHECK_FUNC(dabt_cm,    ESR_ELx_CM);
>> +
>> +ESR_DECLARE_GET_FUNC(class,        ESR_ELx_EC_MASK,      ESR_ELx_EC_SHIFT);
>> +ESR_DECLARE_GET_FUNC(fault,        ESR_ELx_FSC,          0);
>> +ESR_DECLARE_GET_FUNC(fault_type,   ESR_ELx_FSC_TYPE,     0);
>> +ESR_DECLARE_GET_FUNC(condition,    ESR_ELx_COND_MASK,    ESR_ELx_COND_SHIFT);
>> +ESR_DECLARE_GET_FUNC(hvc_imm,      ESR_ELx_xVC_IMM_MASK, 0);
>> +ESR_DECLARE_GET_FUNC(dabt_iss_nisv_sanitized,
>> +		     (ESR_ELx_CM | ESR_ELx_WNR | ESR_ELx_FSC), 0);
>> +ESR_DECLARE_GET_FUNC(dabt_rd,      ESR_ELx_SRT_MASK,     ESR_ELx_SRT_SHIFT);
>> +ESR_DECLARE_GET_FUNC(dabt_as,      ESR_ELx_SAS,          ESR_ELx_SAS_SHIFT);
>> +ESR_DECLARE_GET_FUNC(sys_rt,       ESR_ELx_SYS64_ISS_RT_MASK,
>> +				   ESR_ELx_SYS64_ISS_RT_SHIFT);
> 
> I'm really not keen on this, as I think it's abstracting the problem at
> the wrong level, hiding information and making things harder to reason
> about rather than abstracting that.
> 
> I strongly suspect the right thing to do is use FIELD_GET() in-place in
> the functions below, e.g.
> 
>     !!FIELD_GET(esr, ESR_ELx_IL);
> 
> ... rather than:
> 
>     esr_get_il_32bit(esr);
> 
> ... as that avoids the wrapper entirely, minimizing indirection and
> making the codebase simpler to navigate.
> 
> For the cases where we *really* want a helper, i'd rather write those
> out explicitly, e.g.
> 

It will be no difference except to use FIELD_GET() to make the code
more explicit. Maybe I didn't fully understand your comments here.
Please let me know if something like below is what you expect?

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index c9ba0df47f7d..e8294edcd8f4 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -343,7 +343,7 @@ static __always_inline unsigned int kvm_vcpu_dabt_get_as(const struct kvm_vcpu *
  /* This one is not specific to Data Abort */
  static __always_inline bool kvm_vcpu_trap_il_is32bit(const struct kvm_vcpu *vcpu)
  {
-       return !!(kvm_vcpu_get_esr(vcpu) & ESR_ELx_IL);
+       return !!FIELD_GET(kvm_vcpu_get_esr(vcpu), ESR_ELx_IL);
  }

If my understanding is correct, I think we needn't change the code
and this patch can be dropped.

> #define esr_get_hvc_imm(esr)	FIELD_GET(esr, ESR_ELx_xVC_IMM_MASK)
> 
> ... but I'm not sure if we really need those given these are mostly used
> *once* below.
> 

We don't need these for now, but will be needed when the next revision
of async page fault is posted. Lets ignore this requirement for now
because I can revisit it when the async page fault patchset is posted.
That time, we can have accessors defined in esr.h and helpers in
kvm_emulate.h use those accessors. It's similar to what you're suggesting.

#define esr_get_hvc_imm(esr)	FIELD_GET(esr, ESR_ELx_xVC_IMM_MASK)

static inline u32 kvm_vcpu_hvc_get_imm(const struct kvm_vcpu *vcpu)
{
	return esr_get_hvc_imm(kvm_vcpu_get_esr(vcpu));
}


By the way, it's long way to reach that point because I'm still in the
middle of working on virtualizing SDEI currently.

>> +
>>   const char *esr_get_class_string(u32 esr);
>>   #endif /* __ASSEMBLY */
>>   
>> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
>> index c9ba0df47f7d..9337d90c517f 100644
>> --- a/arch/arm64/include/asm/kvm_emulate.h
>> +++ b/arch/arm64/include/asm/kvm_emulate.h
>> @@ -266,12 +266,8 @@ static __always_inline u32 kvm_vcpu_get_esr(const struct kvm_vcpu *vcpu)
>>   
>>   static __always_inline int kvm_vcpu_get_condition(const struct kvm_vcpu *vcpu)
>>   {
>> -	u32 esr = kvm_vcpu_get_esr(vcpu);
>> -
>> -	if (esr & ESR_ELx_CV)
>> -		return (esr & ESR_ELx_COND_MASK) >> ESR_ELx_COND_SHIFT;
>> -
>> -	return -1;
>> +	return esr_is_condition(kvm_vcpu_get_esr(vcpu)) ?
>> +	       esr_get_condition(kvm_vcpu_get_esr(vcpu)) : -1;
>>   }
> 
> Do we really need to change the structure of this code? I thought this
> was purely about decooupling helpers from the vcpu struct. This could
> have stayed as:
> 
> static __always_inline int kvm_vcpu_get_condition(const struct kvm_vcpu *vcpu)
> {
> 	u32 esr = kvm_vcpu_get_esr(vcpu);
> 
> 	if (esr_is_condition(esr))
> 		return esr_get_condition(esr);
> 	
> 	return -1;
> }
> 
> ... or:
> 
> static __always_inline int kvm_vcpu_get_condition(const struct kvm_vcpu *vcpu)
> {
> 	u32 esr = kvm_vcpu_get_esr(vcpu);
> 
> 	if (FEILD_GET(esr, ESR_ELx_CV))
> 		return FIELD_GET(esr, ESR_ELx_COND_MASK);
> 	
> 	return -1;
> }
> 

It's not needed to change the structure of the code, but it does
reduce the lines of codes. It's kind of my personal taste :)

[...]

Thanks,
Gavin

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