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

["Alex Bennée" <alex.bennee@linaro.org>]

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