On 04.10.2013, at 06:26, Bhushan Bharat-R65777 wrote:
>
>
>> -----Original Message-----
>> From: Wood Scott-B07421
>> Sent: Thursday, October 03, 2013 12:04 AM
>> To: Alexander Graf
>> Cc: Bhushan Bharat-R65777; kvm-ppc@vger.kernel.org; k...@vger.kernel.org;
>> Bhushan
>> Bharat-R65777
>> Subject: Re: [PATCH 1/2] kvm/powerpc: rename kvm_hypercall() to
>> epapr_hypercall()
>>
>> On Wed, 2013-10-02 at 19:54 +0200, Alexander Graf wrote:
>>> On 02.10.2013, at 19:49, Scott Wood wrote:
>>>
>>>> On Wed, 2013-10-02 at 19:46 +0200, Alexander Graf wrote:
>>>>> On 02.10.2013, at 19:42, Scott Wood wrote:
>>>>>
>>>>>> On Wed, 2013-10-02 at 19:17 +0200, Alexander Graf wrote:
>>>>>>> On 02.10.2013, at 19:04, Scott Wood wrote:
>>>>>>>
>>>>>>>> On Wed, 2013-10-02 at 18:53 +0200, Alexander Graf wrote:
>>>>>>>>> On 02.10.2013, at 18:40, Scott Wood wrote:
>>>>>>>>>
>>>>>>>>>> On Wed, 2013-10-02 at 16:19 +0200, Alexander Graf wrote:
>>>>>>>>>>> Won't this break when CONFIG_EPAPR_PARAVIRT=n? We wouldn't have
>> epapr_hcalls.S compiled into the code base then and the bl above would
>> reference
>> an unknown function.
>>>>>>>>>>
>>>>>>>>>> KVM_GUEST selects EPAPR_PARAVIRT.
>>>>>>>>>
>>>>>>>>> But you can not select KVM_GUEST and still call these inline
>>>>>>>>> functions,
>> no?
>>>>>>>>
>>>>>>>> No.
>>>>>>>>
>>>>>>>>> Like kvm_arch_para_features().
>>>>>>>>
>>>>>>>> Where does that get called without KVM_GUEST?
>>>>>>>>
>>>>>>>> How would that work currently, with the call to kvm_hypercall()
>>>>>>>> in arch/powerpc/kernel/kvm.c (which calls epapr_hypercall, BTW)?
>>>>>>>
>>>>>>> It wouldn't ever get called because kvm_hypercall() ends up always
>> returning EV_UNIMPLEMENTED when #ifndef CONFIG_KVM_GUEST.
>>>>>>
>>>>>> OK, so the objection is to removing that stub? Where would we
>>>>>> actually want to call this without knowing that KVM_GUEST or
>>>>>> EPAPR_PARAVIRT are enabled?
>>>>>
>>>>> In probing code. I usually prefer
>>>>>
>>>>> if (kvm_feature_available(X)) {
>>>>> ...
>>>>> }
>>>>>
>>>>> over
>>>>>
>>>>> #ifdef CONFIG_KVM_GUEST
>>>>> if (kvm_feature_available(X)) {
>>>>> ...
>>>>> }
>>>>> #endif
>>>>>
>>>>> at least when I can avoid it. With the current code the compiler would be
>> smart enough to just optimize out the complete branch.
>>>>
>>>> Sure. My point is, where would you be calling that where the entire
>>>> file isn't predicated on (or selecting) CONFIG_KVM_GUEST or similar?
>>>>
>>>> We don't do these stubs for every single function in the kernel --
>>>> only ones where the above is a reasonable use case.
>>>
>>> Yeah, I'm fine on dropping it, but we need to make that a conscious decision
>> and verify that no caller relies on it.
>>
>> kvm_para_has_feature() is called from arch/powerpc/kernel/kvm.c,
>> arch/x86/kernel/kvm.c, and arch/x86/kernel/kvmclock.c, all of which are
>> enabled
>> by CONFIG_KVM_GUEST.
>>
>> I did find one example of kvm_para_available() being used in an unexpected
>> place
>> -- sound/pci/intel8x0.c. It defines its own non-CONFIG_KVM_GUEST stub, even
>> though x86 defines kvm_para_available() using inline CPUID stuff which should
>> work without CONFIG_KVM_GUEST.
>> I'm not sure why it even needs to do that, though -- shouldn't the subsequent
>> PCI subsystem vendor/device check should be sufficient? No hypercalls are
>> involved.
>>
>> That said, the possibility that some random driver might want to make use of
>> paravirt features is a decent argument for keeping the stub.
>>
>
> I am not sure where we are agreeing on?
> Do we want to remove the stub in arch/powerpc/include/asm/kvm_para.h ? as
> there is no caller without KVM_GUEST and in future caller ensure this to be
> called only from code selected by KVM_GUEST?
>
> Or let this stub stay to avoid any random driver calling this ?
I think the most reasonable way forward is to add a stub for non-CONFIG_EPAPR
to the epapr code, then replace the kvm bits with generic epapr bits (which
your patches already do).
With that we should be 100% equivalent to today's code, just with a lot less
lines of code :).
Alex
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