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

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Message-ID: <55702D63.1050402@arm.com>
Date:	Thu, 04 Jun 2015 11:50:11 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Alex Bennée <alex.bennee@...aro.org>
CC:	Christoffer Dall <christoffer.dall@...aro.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: arm64: fix misleading comments in save/restore

On 04/06/15 11:46, Alex Bennée wrote:
> 
> Marc Zyngier <marc.zyngier@....com> writes:
> 
>> On 04/06/15 11:20, Alex Bennée wrote:
>>>
>>> Marc Zyngier <marc.zyngier@....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@...aro.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...
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