[PATCH] KVM: arm64: fix misleading comments in save/restore

[PATCH] KVM: arm64: fix misleading comments in save/restore

[Alex Bennée]

The elr_el2 and spsr_el2 registers in fact contain the processor state
before entry into the hypervisor code. In the case of guest state it
could be in either el0 or el1.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 arch/arm64/kvm/hyp.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
index d755922..1940a4c 100644
--- a/arch/arm64/kvm/hyp.S
+++ b/arch/arm64/kvm/hyp.S
@@ -50,8 +50,8 @@
 	stp	x29, lr, [x3, #80]
 
 	mrs	x19, sp_el0
-	mrs	x20, elr_el2		// EL1 PC
-	mrs	x21, spsr_el2		// EL1 pstate
+	mrs	x20, elr_el2		// PC before hyp entry
+	mrs	x21, spsr_el2		// pstate before hyp entry
 
 	stp	x19, x20, [x3, #96]
 	str	x21, [x3, #112]
@@ -82,8 +82,8 @@
 	ldr	x21, [x3, #16]
 
 	msr	sp_el0, x19
-	msr	elr_el2, x20 				// EL1 PC
-	msr	spsr_el2, x21 				// EL1 pstate
+	msr	elr_el2, x20 		// PC to restore
+	msr	spsr_el2, x21 		// pstate to restore
 
 	add	x3, x2, #CPU_XREG_OFFSET(19)
 	ldp	x19, x20, [x3]
-- 
2.4.1

Re: [PATCH] KVM: arm64: fix misleading comments in save/restore

[Christoffer Dall]

On Thu, May 28, 2015 at 10:43:06AM +0100, Alex Bennée wrote:
> The elr_el2 and spsr_el2 registers in fact contain the processor state
> before entry into the hypervisor code.

be careful with your use of the hypervisor, in the KVM design the
hypervisor is split across EL1 and EL2.

> In the case of guest state it
> could be in either el0 or el1.

true

> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  arch/arm64/kvm/hyp.S | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
> index d755922..1940a4c 100644
> --- a/arch/arm64/kvm/hyp.S
> +++ b/arch/arm64/kvm/hyp.S
> @@ -50,8 +50,8 @@
>  	stp	x29, lr, [x3, #80]
>  
>  	mrs	x19, sp_el0
> -	mrs	x20, elr_el2		// EL1 PC
> -	mrs	x21, spsr_el2		// EL1 pstate
> +	mrs	x20, elr_el2		// PC before hyp entry
> +	mrs	x21, spsr_el2		// pstate before hyp entry
>  
>  	stp	x19, x20, [x3, #96]
>  	str	x21, [x3, #112]
> @@ -82,8 +82,8 @@
>  	ldr	x21, [x3, #16]
>  
>  	msr	sp_el0, x19
> -	msr	elr_el2, x20 				// EL1 PC
> -	msr	spsr_el2, x21 				// EL1 pstate
> +	msr	elr_el2, x20 		// PC to restore
> +	msr	spsr_el2, x21 		// pstate to restore

I don't feel like 'to restore' is much more meaningful here.

I would actually vote for removin the comments all together, since one
should really understand the code as opposed to the comments when
reading this kind of stuff.

Meh, I'm not sure.  Your patch is definitely better than doing nothing.

Marc?

-Christoffer

Re: [PATCH] KVM: arm64: fix misleading comments in save/restore

[Marc Zyngier]

On 04/06/15 10:34, Christoffer Dall wrote:
> On Thu, May 28, 2015 at 10:43:06AM +0100, Alex Bennée wrote:
>> The elr_el2 and spsr_el2 registers in fact contain the processor state
>> before entry into the hypervisor code.
> 
> be careful with your use of the hypervisor, in the KVM design the
> hypervisor is split across EL1 and EL2.
> 
>> In the case of guest state it
>> could be in either el0 or el1.
> 
> true
> 
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> ---
>>  arch/arm64/kvm/hyp.S | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>> index d755922..1940a4c 100644
>> --- a/arch/arm64/kvm/hyp.S
>> +++ b/arch/arm64/kvm/hyp.S
>> @@ -50,8 +50,8 @@
>>  	stp	x29, lr, [x3, #80]
>>  
>>  	mrs	x19, sp_el0
>> -	mrs	x20, elr_el2		// EL1 PC
>> -	mrs	x21, spsr_el2		// EL1 pstate
>> +	mrs	x20, elr_el2		// PC before hyp entry
>> +	mrs	x21, spsr_el2		// pstate before hyp entry
>>  
>>  	stp	x19, x20, [x3, #96]
>>  	str	x21, [x3, #112]
>> @@ -82,8 +82,8 @@
>>  	ldr	x21, [x3, #16]
>>  
>>  	msr	sp_el0, x19
>> -	msr	elr_el2, x20 				// EL1 PC
>> -	msr	spsr_el2, x21 				// EL1 pstate
>> +	msr	elr_el2, x20 		// PC to restore
>> +	msr	spsr_el2, x21 		// pstate to restore
> 
> I don't feel like 'to restore' is much more meaningful here.
> 
> I would actually vote for removin the comments all together, since one
> should really understand the code as opposed to the comments when
> reading this kind of stuff.
> 
> Meh, I'm not sure.  Your patch is definitely better than doing nothing.
> 
> Marc?

While I definitely agree that people should pay more attention to the
code rather than blindly trusting comments, I still think there is some
value in disambiguating the exception entry/return, because this bit of
code assumes some intimate knowledge of the ARMv8 exception model.

As for the comments themselves, I'd rather have some wording that
clearly indicate that we're dealing with guest information, i.e:

	mrs	x20, elr_el2		// Guest PC
	mrs	x21, spsr_el2		// Guest pstate

(and the same for the exception return). The "before hyp entry" and "to
restore" are not really useful (all the registers we are
saving/restoring fall into these categories). What I wanted to convey
here was that despite using an EL2 register, we are dealing with guest
registers.

Would this address your concerns?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

Re: [PATCH] KVM: arm64: fix misleading comments in save/restore

[Alex Bennée]

Marc Zyngier <marc.zyngier@arm.com> writes:

> On 04/06/15 10:34, Christoffer Dall wrote:
>> On Thu, May 28, 2015 at 10:43:06AM +0100, Alex Bennée wrote:
>>> The elr_el2 and spsr_el2 registers in fact contain the processor state
>>> before entry into the hypervisor code.
>> 
>> be careful with your use of the hypervisor, in the KVM design the
>> hypervisor is split across EL1 and EL2.

"before entry into EL2."

>> 
>>> In the case of guest state it
>>> could be in either el0 or el1.
>> 
>> true
>> 
>>>
>>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>>> ---
>>>  arch/arm64/kvm/hyp.S | 8 ++++----
>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>>> index d755922..1940a4c 100644
>>> --- a/arch/arm64/kvm/hyp.S
>>> +++ b/arch/arm64/kvm/hyp.S
>>> @@ -50,8 +50,8 @@
>>>  	stp	x29, lr, [x3, #80]
>>>  
>>>  	mrs	x19, sp_el0
>>> -	mrs	x20, elr_el2		// EL1 PC
>>> -	mrs	x21, spsr_el2		// EL1 pstate
>>> +	mrs	x20, elr_el2		// PC before hyp entry
>>> +	mrs	x21, spsr_el2		// pstate before hyp entry
>>>  
>>>  	stp	x19, x20, [x3, #96]
>>>  	str	x21, [x3, #112]
>>> @@ -82,8 +82,8 @@
>>>  	ldr	x21, [x3, #16]
>>>  
>>>  	msr	sp_el0, x19
>>> -	msr	elr_el2, x20 				// EL1 PC
>>> -	msr	spsr_el2, x21 				// EL1 pstate
>>> +	msr	elr_el2, x20 		// PC to restore
>>> +	msr	spsr_el2, x21 		// pstate to restore
>> 
>> I don't feel like 'to restore' is much more meaningful here.
>> 
>> I would actually vote for removin the comments all together, since one
>> should really understand the code as opposed to the comments when
>> reading this kind of stuff.
>> 
>> Meh, I'm not sure.  Your patch is definitely better than doing nothing.
>> 
>> Marc?
>
> While I definitely agree that people should pay more attention to the
> code rather than blindly trusting comments, I still think there is some
> value in disambiguating the exception entry/return, because this bit of
> code assumes some intimate knowledge of the ARMv8 exception model.
>
> As for the comments themselves, I'd rather have some wording that
> clearly indicate that we're dealing with guest information, i.e:
>
> 	mrs	x20, elr_el2		// Guest PC
> 	mrs	x21, spsr_el2		// Guest pstate
>
> (and the same for the exception return). The "before hyp entry" and "to
> restore" are not really useful (all the registers we are
> saving/restoring fall into these categories). What I wanted to convey
> here was that despite using an EL2 register, we are dealing with guest
> registers.

Which would be great it we were. However the code is used to
save/restore the host context as well as the guest context hence my
weasely words. 

>
> Would this address your concerns?
>
> Thanks,
>
> 	M.

-- 
Alex Bennée

Re: [PATCH] KVM: arm64: fix misleading comments in save/restore

[Marc Zyngier]

On 04/06/15 11:20, Alex Bennée wrote:
> 
> Marc Zyngier <marc.zyngier@arm.com> writes:
> 
>> On 04/06/15 10:34, Christoffer Dall wrote:
>>> On Thu, May 28, 2015 at 10:43:06AM +0100, Alex Bennée wrote:
>>>> The elr_el2 and spsr_el2 registers in fact contain the processor state
>>>> before entry into the hypervisor code.
>>>
>>> be careful with your use of the hypervisor, in the KVM design the
>>> hypervisor is split across EL1 and EL2.
> 
> "before entry into EL2."
> 
>>>
>>>> In the case of guest state it
>>>> could be in either el0 or el1.
>>>
>>> true
>>>
>>>>
>>>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>>>> ---
>>>>  arch/arm64/kvm/hyp.S | 8 ++++----
>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>>>> index d755922..1940a4c 100644
>>>> --- a/arch/arm64/kvm/hyp.S
>>>> +++ b/arch/arm64/kvm/hyp.S
>>>> @@ -50,8 +50,8 @@
>>>>  	stp	x29, lr, [x3, #80]
>>>>  
>>>>  	mrs	x19, sp_el0
>>>> -	mrs	x20, elr_el2		// EL1 PC
>>>> -	mrs	x21, spsr_el2		// EL1 pstate
>>>> +	mrs	x20, elr_el2		// PC before hyp entry
>>>> +	mrs	x21, spsr_el2		// pstate before hyp entry
>>>>  
>>>>  	stp	x19, x20, [x3, #96]
>>>>  	str	x21, [x3, #112]
>>>> @@ -82,8 +82,8 @@
>>>>  	ldr	x21, [x3, #16]
>>>>  
>>>>  	msr	sp_el0, x19
>>>> -	msr	elr_el2, x20 				// EL1 PC
>>>> -	msr	spsr_el2, x21 				// EL1 pstate
>>>> +	msr	elr_el2, x20 		// PC to restore
>>>> +	msr	spsr_el2, x21 		// pstate to restore
>>>
>>> I don't feel like 'to restore' is much more meaningful here.
>>>
>>> I would actually vote for removin the comments all together, since one
>>> should really understand the code as opposed to the comments when
>>> reading this kind of stuff.
>>>
>>> Meh, I'm not sure.  Your patch is definitely better than doing nothing.
>>>
>>> Marc?
>>
>> While I definitely agree that people should pay more attention to the
>> code rather than blindly trusting comments, I still think there is some
>> value in disambiguating the exception entry/return, because this bit of
>> code assumes some intimate knowledge of the ARMv8 exception model.
>>
>> As for the comments themselves, I'd rather have some wording that
>> clearly indicate that we're dealing with guest information, i.e:
>>
>> 	mrs	x20, elr_el2		// Guest PC
>> 	mrs	x21, spsr_el2		// Guest pstate
>>
>> (and the same for the exception return). The "before hyp entry" and "to
>> restore" are not really useful (all the registers we are
>> saving/restoring fall into these categories). What I wanted to convey
>> here was that despite using an EL2 register, we are dealing with guest
>> registers.
> 
> Which would be great it we were. However the code is used to
> save/restore the host context as well as the guest context hence my
> weasely words. 

Gahhh. You're right. I'm spending too much time on the VHE code these
days. Guess I'll stick to the weasel words then. Can you respin it with
Christoffer's comment addressed?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

Re: [PATCH] KVM: arm64: fix misleading comments in save/restore

[Alex Bennée]

Marc Zyngier <marc.zyngier@arm.com> writes:

> On 04/06/15 11:20, Alex Bennée wrote:
>> 
>> Marc Zyngier <marc.zyngier@arm.com> writes:
>> 
>>> On 04/06/15 10:34, Christoffer Dall wrote:
>>>> On Thu, May 28, 2015 at 10:43:06AM +0100, Alex Bennée wrote:
>>>>> The elr_el2 and spsr_el2 registers in fact contain the processor state
>>>>> before entry into the hypervisor code.
>>>>
>>>> be careful with your use of the hypervisor, in the KVM design the
>>>> hypervisor is split across EL1 and EL2.
>> 
>> "before entry into EL2."
>> 
>>>>
>>>>> In the case of guest state it
>>>>> could be in either el0 or el1.
>>>>
>>>> true
>>>>
>>>>>
>>>>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>>>>> ---
>>>>>  arch/arm64/kvm/hyp.S | 8 ++++----
>>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>>>>> index d755922..1940a4c 100644
>>>>> --- a/arch/arm64/kvm/hyp.S
>>>>> +++ b/arch/arm64/kvm/hyp.S
>>>>> @@ -50,8 +50,8 @@
>>>>>  	stp	x29, lr, [x3, #80]
>>>>>  
>>>>>  	mrs	x19, sp_el0
>>>>> -	mrs	x20, elr_el2		// EL1 PC
>>>>> -	mrs	x21, spsr_el2		// EL1 pstate
>>>>> +	mrs	x20, elr_el2		// PC before hyp entry
>>>>> +	mrs	x21, spsr_el2		// pstate before hyp entry
>>>>>  
>>>>>  	stp	x19, x20, [x3, #96]
>>>>>  	str	x21, [x3, #112]
>>>>> @@ -82,8 +82,8 @@
>>>>>  	ldr	x21, [x3, #16]
>>>>>  
>>>>>  	msr	sp_el0, x19
>>>>> -	msr	elr_el2, x20 				// EL1 PC
>>>>> -	msr	spsr_el2, x21 				// EL1 pstate
>>>>> +	msr	elr_el2, x20 		// PC to restore
>>>>> +	msr	spsr_el2, x21 		// pstate to restore
>>>>
>>>> I don't feel like 'to restore' is much more meaningful here.
>>>>
>>>> I would actually vote for removin the comments all together, since one
>>>> should really understand the code as opposed to the comments when
>>>> reading this kind of stuff.
>>>>
>>>> Meh, I'm not sure.  Your patch is definitely better than doing nothing.
>>>>
>>>> Marc?
>>>
>>> While I definitely agree that people should pay more attention to the
>>> code rather than blindly trusting comments, I still think there is some
>>> value in disambiguating the exception entry/return, because this bit of
>>> code assumes some intimate knowledge of the ARMv8 exception model.
>>>
>>> As for the comments themselves, I'd rather have some wording that
>>> clearly indicate that we're dealing with guest information, i.e:
>>>
>>> 	mrs	x20, elr_el2		// Guest PC
>>> 	mrs	x21, spsr_el2		// Guest pstate
>>>
>>> (and the same for the exception return). The "before hyp entry" and "to
>>> restore" are not really useful (all the registers we are
>>> saving/restoring fall into these categories). What I wanted to convey
>>> here was that despite using an EL2 register, we are dealing with guest
>>> registers.
>> 
>> Which would be great it we were. However the code is used to
>> save/restore the host context as well as the guest context hence my
>> weasely words. 
>
> Gahhh. You're right. I'm spending too much time on the VHE code these
> days. Guess I'll stick to the weasel words then. Can you respin it with
> Christoffer's comment addressed?

Sure. Do you want it separated from the guest debug series or will you
be happy to take it with it when ready?

>
> Thanks,
>
> 	M.

-- 
Alex Bennée

Re: [PATCH] KVM: arm64: fix misleading comments in save/restore

[Marc Zyngier]

On 04/06/15 11:46, Alex Bennée wrote:
> 
> Marc Zyngier <marc.zyngier@arm.com> writes:
> 
>> On 04/06/15 11:20, Alex Bennée wrote:
>>>
>>> Marc Zyngier <marc.zyngier@arm.com> writes:
>>>
>>>> On 04/06/15 10:34, Christoffer Dall wrote:
>>>>> On Thu, May 28, 2015 at 10:43:06AM +0100, Alex Bennée wrote:
>>>>>> The elr_el2 and spsr_el2 registers in fact contain the processor state
>>>>>> before entry into the hypervisor code.
>>>>>
>>>>> be careful with your use of the hypervisor, in the KVM design the
>>>>> hypervisor is split across EL1 and EL2.
>>>
>>> "before entry into EL2."
>>>
>>>>>
>>>>>> In the case of guest state it
>>>>>> could be in either el0 or el1.
>>>>>
>>>>> true
>>>>>
>>>>>>
>>>>>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>>>>>> ---
>>>>>>  arch/arm64/kvm/hyp.S | 8 ++++----
>>>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>>>>>> index d755922..1940a4c 100644
>>>>>> --- a/arch/arm64/kvm/hyp.S
>>>>>> +++ b/arch/arm64/kvm/hyp.S
>>>>>> @@ -50,8 +50,8 @@
>>>>>>  	stp	x29, lr, [x3, #80]
>>>>>>  
>>>>>>  	mrs	x19, sp_el0
>>>>>> -	mrs	x20, elr_el2		// EL1 PC
>>>>>> -	mrs	x21, spsr_el2		// EL1 pstate
>>>>>> +	mrs	x20, elr_el2		// PC before hyp entry
>>>>>> +	mrs	x21, spsr_el2		// pstate before hyp entry
>>>>>>  
>>>>>>  	stp	x19, x20, [x3, #96]
>>>>>>  	str	x21, [x3, #112]
>>>>>> @@ -82,8 +82,8 @@
>>>>>>  	ldr	x21, [x3, #16]
>>>>>>  
>>>>>>  	msr	sp_el0, x19
>>>>>> -	msr	elr_el2, x20 				// EL1 PC
>>>>>> -	msr	spsr_el2, x21 				// EL1 pstate
>>>>>> +	msr	elr_el2, x20 		// PC to restore
>>>>>> +	msr	spsr_el2, x21 		// pstate to restore
>>>>>
>>>>> I don't feel like 'to restore' is much more meaningful here.
>>>>>
>>>>> I would actually vote for removin the comments all together, since one
>>>>> should really understand the code as opposed to the comments when
>>>>> reading this kind of stuff.
>>>>>
>>>>> Meh, I'm not sure.  Your patch is definitely better than doing nothing.
>>>>>
>>>>> Marc?
>>>>
>>>> While I definitely agree that people should pay more attention to the
>>>> code rather than blindly trusting comments, I still think there is some
>>>> value in disambiguating the exception entry/return, because this bit of
>>>> code assumes some intimate knowledge of the ARMv8 exception model.
>>>>
>>>> As for the comments themselves, I'd rather have some wording that
>>>> clearly indicate that we're dealing with guest information, i.e:
>>>>
>>>> 	mrs	x20, elr_el2		// Guest PC
>>>> 	mrs	x21, spsr_el2		// Guest pstate
>>>>
>>>> (and the same for the exception return). The "before hyp entry" and "to
>>>> restore" are not really useful (all the registers we are
>>>> saving/restoring fall into these categories). What I wanted to convey
>>>> here was that despite using an EL2 register, we are dealing with guest
>>>> registers.
>>>
>>> Which would be great it we were. However the code is used to
>>> save/restore the host context as well as the guest context hence my
>>> weasely words. 
>>
>> Gahhh. You're right. I'm spending too much time on the VHE code these
>> days. Guess I'll stick to the weasel words then. Can you respin it with
>> Christoffer's comment addressed?
> 
> Sure. Do you want it separated from the guest debug series or will you
> be happy to take it with it when ready?

I'll take it now, no need to wait on the whole debug series to fix this.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...