Re: [PATCH v3 2/2] KVM: x86: directly use kvm_make_request again

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 09/19/2014 08:25 PM, Radim Krčmář wrote:

>>>   * Returns 1 to let __vcpu_run() continue the guest execution loop without
>>>   * exiting to the userspace.  Otherwise, the value will be returned to the
>>> @@ -6018,8 +6024,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>>  		if (kvm_check_request(KVM_REQ_MMU_SYNC, vcpu))
>>>  			kvm_mmu_sync_roots(vcpu);
>>>  		if (kvm_check_request(KVM_REQ_TLB_FLUSH, vcpu)) {
>>> -			++vcpu->stat.tlb_flush;
>>> -			kvm_x86_ops->tlb_flush(vcpu);
>>> +			kvm_vcpu_flush_tlb(vcpu);
>>
>> NACK!
>>
>> Do not understand why you have to introduce a meaningful name
>> here - it's used just inner a function, which can not help to
>> improve a readability of the code at all.
> 
> I prefer the new hunk
>  - it makes the parent function simpler (not everyone wants to read how
>    we do tlb flushes when looking at vcpu_enter_guest)

Using one line instead of two lines does not simplify parent function much.

>  - the function is properly named

kvm_x86_ops->tlb_flush(vcpu) is also a good hit to tell the reader it is
doing tlb flush. :)

>  - we do a similar thing with kvm_gen_kvmclock_update

I understand this raw-bit-set style is largely used in current kvm code,
however, it does not mean it's a best way do it. It may be turned off
someday as it is be used in more and more places.

Anyway, the meaningful name wrapping raw-bit-set is a right direction
and let's keep this right direction.

> 
>> What i suggested is renaming kvm_mmu_flush_tlb() since it's a
>> API used in multiple files - a good name helps developer to
>> know what it's doing and definitely easier typing.
> 
> I think it is a good idea.
> The proposed name is definitely better than what we have now.
> 
> You can see reasons that led me to prefer raw request below.
> (Preferring something else is no way means that I'm against your idea.)

I understand that, Radim! :)

> 
> ---
> I'm always trying to reach some ideal code in my mind, which makes me
> seemingly oppose good proposals because I see how it could be even
> better ...  and I opt not to do them.
> (Pushing minor refactoring patches upstream is hard!)
> 
> My issues with kvm_mmu_flush_tlb:
> 
>  - 'kvm_flush_remote_tlbs()' calls tlb request directly;
>     our wrapper thus cannot be extended with features, which makes it a
>     poor abstraction

kvm_flush_remote_tlbs does not only set tlb request but also handles memory
order and syncs the tlb state.

I guess you wanted to say kvm_mmu_flush_tlb here, it is a API name and let
it be easily used in other files. It's not worth committing a patch doing
nothing except reverting the meaningful name.

>  - we don't do this for other requests

See above.

>  - direct request isn't absolutely horrible to read and write
>    (I totally agree that it is bad.)
>  - we call one function 'kvm_mmu_flush_tlb()' and the second one
>    'kvm_flush_remote_tlbs()' and I'd need to look why

Yeah, this is why i suggested to rename kvm_mmu_flush_tlb since which clarifies
things better:
- kvm_flush_remote_tlbs: flush tlb in all vcpus
- kvm_vcpu_flush_tlb: only flush tlb on the vcpu specified by @vcpu.

> 
> Which is why just removing it solves more problems for me :)

Thank you for raising this question and letting me know the patch's history. :)