[PATCH] KVM: arm64: fix misleading comments in save/restore
Alex Bennée
alex.bennee at linaro.org
Thu Jun 4 03:20:33 PDT 2015
Marc Zyngier <marc.zyngier at 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 at 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
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