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

[Tianjia Zhang]

On 2020/4/26 20:59, Thomas Huth wrote:
> 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@xxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > > On 2020/4/23 0:04, Cornelia Huck wrote:
> > > > > > On Wed, 22 Apr 2020 17:58:04 +0200
> > > > > > Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
> > > > > >    
> > > > > > > On 22.04.20 15:45, Cornelia Huck wrote:
> > > > > > > > On Wed, 22 Apr 2020 20:58:04 +0800
> > > > > > > > Tianjia Zhang <tianjia.zhang@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> > > > > > > > > ---
> > > > > > > > >  ÂÂ arch/s390/kvm/kvm-s390.c | 37 ++++++++++++++++++++++---------------
> > > > > > > > >  ÂÂ 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)
> > > > > > > > >  ÂÂÂÂÂÂ return rc;
> > > > > > > > >  ÂÂ }
> > > > > > > > >  ÂÂ -static void sync_regs_fmt2(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> > > > > > > > > +static void sync_regs_fmt2(struct kvm_vcpu *vcpu)
> > > > > > > > >  ÂÂ {
> > > > > > > > > +ÂÂÂ struct kvm_run *kvm_run = vcpu->run;
> > > > > > > > >  ÂÂÂÂÂÂ struct runtime_instr_cb *riccb;
> > > > > > > > >  ÂÂÂÂÂÂ struct gs_cb *gscb;
> > > > > > > > >  ÂÂ @@ -4235,7 +4236,7 @@ static void sync_regs_fmt2(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> > > > > > > > >  ÂÂÂÂÂÂÂÂÂÂ }
> > > > > > > > >  ÂÂÂÂÂÂÂÂÂÂ if (vcpu->arch.gs_enabled) {
> > > > > > > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ current->thread.gs_cb = (struct gs_cb *)
> > > > > > > > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ &vcpu->run->s.regs.gscb;
> > > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ &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

If the compiler is smart enough, this place can be automatically 
optimized, but we can't just rely on the compiler, if not? This requires 
a trade-off between code cleanliness readability and breaking git blame.
In addition, I have removed the changes here and sent a v4 patch. Please 
also help review it.

Thanks and best,
Tianjia