Re: [PATCH v2 1/7] KVM: s390: clean up redundant 'kvm_run' parameters

[Thomas Huth <thuth@redhat.com>]

On 23/04/2020 13.00, Christian Borntraeger wrote:
> 
> 
> On 23.04.20 12:58, Tianjia Zhang wrote:
>>
>>
>> On 2020/4/23 18:39, Cornelia Huck wrote:
>>> On Thu, 23 Apr 2020 11:01:43 +0800
>>> Tianjia Zhang <tianjia.zhang@linux.alibaba.com> wrote:
>>>
>>>> On 2020/4/23 0:04, Cornelia Huck wrote:
>>>>> On Wed, 22 Apr 2020 17:58:04 +0200
>>>>> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>>>>>   
>>>>>> On 22.04.20 15:45, Cornelia Huck wrote:
>>>>>>> On Wed, 22 Apr 2020 20:58:04 +0800
>>>>>>> Tianjia Zhang <tianjia.zhang@linux.alibaba.com> wrote:
>>>>>>>      
>>>>>>>> In the current kvm version, 'kvm_run' has been included in the 'kvm_vcpu'
>>>>>>>> structure. Earlier than historical reasons, many kvm-related function
>>>>>>>
>>>>>>> s/Earlier than/For/ ?
>>>>>>>      
>>>>>>>> parameters retain the 'kvm_run' and 'kvm_vcpu' parameters at the same time.
>>>>>>>> This patch does a unified cleanup of these remaining redundant parameters.
>>>>>>>>
>>>>>>>> Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
>>>>>>>> ---
>>>>>>>>    arch/s390/kvm/kvm-s390.c | 37 ++++++++++++++++++++++---------------
>>>>>>>>    1 file changed, 22 insertions(+), 15 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>>>>> index e335a7e5ead7..d7bb2e7a07ff 100644
>>>>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>>>>> @@ -4176,8 +4176,9 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
>>>>>>>>        return rc;
>>>>>>>>    }
>>>>>>>>    -static void sync_regs_fmt2(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>>>>>>>> +static void sync_regs_fmt2(struct kvm_vcpu *vcpu)
>>>>>>>>    {
>>>>>>>> +    struct kvm_run *kvm_run = vcpu->run;
>>>>>>>>        struct runtime_instr_cb *riccb;
>>>>>>>>        struct gs_cb *gscb;
>>>>>>>>    @@ -4235,7 +4236,7 @@ static void sync_regs_fmt2(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>>>>>>>>            }
>>>>>>>>            if (vcpu->arch.gs_enabled) {
>>>>>>>>                current->thread.gs_cb = (struct gs_cb *)
>>>>>>>> -                        &vcpu->run->s.regs.gscb;
>>>>>>>> +                        &kvm_run->s.regs.gscb;
>>>>>>>
>>>>>>> Not sure if these changes (vcpu->run-> => kvm_run->) are really worth
>>>>>>> it. (It seems they amount to at least as much as the changes advertised
>>>>>>> in the patch description.)
>>>>>>>
>>>>>>> Other opinions?
>>>>>>
>>>>>> Agreed. It feels kind of random. Maybe just do the first line (move kvm_run from the
>>>>>> function parameter list into the variable declaration)? Not sure if this is better.
>>>>>>   
>>>>>
>>>>> There's more in this patch that I cut... but I think just moving
>>>>> kvm_run from the parameter list would be much less disruptive.
>>>>>    
>>>>
>>>> I think there are two kinds of code(`vcpu->run->` and `kvm_run->`), but
>>>> there will be more disruptive, not less.
>>>
>>> I just fail to see the benefit; sure, kvm_run-> is convenient, but the
>>> current code is just fine, and any rework should be balanced against
>>> the cost (e.g. cluttering git annotate).
>>>
>>
>> cluttering git annotate ? Does it mean Fix xxxx ("comment"). Is it possible to solve this problem by splitting this patch?
> 
> No its about breaking git blame (and bugfix backports) for just a cosmetic improvement.

It could be slightly more than a cosmetic improvement (depending on the
smartness of the compiler): vcpu->run-> are two dereferences, while
kvm_run-> is only one dereference. So it could be slightly more compact
and faster code.

 Thomas