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 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