linux-kernel - Re: [PATCH v3 9/9] LoongArch: KVM: INTC: Add address alignment check

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Message-ID: <CAAhV-H7Wnk7j1ukDLT+KZ6+tJuxMFv5qG-YGsJsXfB=2-eC=Ow@mail.gmail.com>
Date: Sat, 21 Jun 2025 21:04:14 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Bibo Mao <maobibo@...ngson.cn>, Tianrui Zhao <zhaotianrui@...ngson.cn>, 
	Xianglai Li <lixianglai@...ngson.cn>, kvm@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 9/9] LoongArch: KVM: INTC: Add address alignment check

Hi, David,

On Sat, Jun 21, 2025 at 7:21 PM David Laight
<david.laight.linux@...il.com> wrote:
>
> On Thu, 19 Jun 2025 16:47:22 +0800
> Huacai Chen <chenhuacai@...nel.org> wrote:
>
> > Hi, Bibo,
> >
> > On Wed, Jun 11, 2025 at 9:51 AM Bibo Mao <maobibo@...ngson.cn> wrote:
> > >
> > > IOCSR instruction supports 1/2/4/8 bytes access, the address should
> > > be naturally aligned with its access size. Here address alignment
> > > check is added in eiointc kernel emulation.
> > >
> > > At the same time len must be 1/2/4/8 bytes from iocsr exit emulation
> > > function kvm_emu_iocsr(), remove the default case in switch case
> > > statements.
> > Robust code doesn't depend its callers do things right, so I suggest
> > keeping the default case, which means we just add the alignment check
> > here.
>
> kernel code generally relies on callers to DTRT - except for values
> that come from userspace.
>
> Otherwise you get unreadable and slow code that continuously checks
> for things that can't happen.
Generally you are right - but this patch is not the case.

Adding a "default" case here doesn't make code slower or unreadable,
and the code becomes more robust.

Huacai

>
>         David
>
> >
> > And I think this patch should also Cc stable and add a Fixes tag.
> >
> >
> > Huacai
> >
> > >
> > > Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
> > > ---
> > >  arch/loongarch/kvm/intc/eiointc.c | 21 +++++++++++++--------
> > >  1 file changed, 13 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/loongarch/kvm/intc/eiointc.c b/arch/loongarch/kvm/intc/eiointc.c
> > > index 8b0d9376eb54..4e9d12300cc4 100644
> > > --- a/arch/loongarch/kvm/intc/eiointc.c
> > > +++ b/arch/loongarch/kvm/intc/eiointc.c
> > > @@ -311,6 +311,12 @@ static int kvm_eiointc_read(struct kvm_vcpu *vcpu,
> > >                 return -EINVAL;
> > >         }
> > >
> > > +       /* len must be 1/2/4/8 from function kvm_emu_iocsr() */
> > > +       if (addr & (len - 1)) {
> > > +               kvm_err("%s: eiointc not aligned addr %llx len %d\n", __func__, addr, len);
> > > +               return -EINVAL;
> > > +       }
> > > +
> > >         vcpu->stat.eiointc_read_exits++;
> > >         spin_lock_irqsave(&eiointc->lock, flags);
> > >         switch (len) {
> > > @@ -323,12 +329,9 @@ static int kvm_eiointc_read(struct kvm_vcpu *vcpu,
> > >         case 4:
> > >                 ret = loongarch_eiointc_readl(vcpu, eiointc, addr, val);
> > >                 break;
> > > -       case 8:
> > > +       default:
> > >                 ret = loongarch_eiointc_readq(vcpu, eiointc, addr, val);
> > >                 break;
> > > -       default:
> > > -               WARN_ONCE(1, "%s: Abnormal address access: addr 0x%llx, size %d\n",
> > > -                                               __func__, addr, len);
> > >         }
> > >         spin_unlock_irqrestore(&eiointc->lock, flags);
> > >
> > > @@ -682,6 +685,11 @@ static int kvm_eiointc_write(struct kvm_vcpu *vcpu,
> > >                 return -EINVAL;
> > >         }
> > >
> > > +       if (addr & (len - 1)) {
> > > +               kvm_err("%s: eiointc not aligned addr %llx len %d\n", __func__, addr, len);
> > > +               return -EINVAL;
> > > +       }
> > > +
> > >         vcpu->stat.eiointc_write_exits++;
> > >         spin_lock_irqsave(&eiointc->lock, flags);
> > >         switch (len) {
> > > @@ -694,12 +702,9 @@ static int kvm_eiointc_write(struct kvm_vcpu *vcpu,
> > >         case 4:
> > >                 ret = loongarch_eiointc_writel(vcpu, eiointc, addr, val);
> > >                 break;
> > > -       case 8:
> > > +       default:
> > >                 ret = loongarch_eiointc_writeq(vcpu, eiointc, addr, val);
> > >                 break;
> > > -       default:
> > > -               WARN_ONCE(1, "%s: Abnormal address access: addr 0x%llx, size %d\n",
> > > -                                               __func__, addr, len);
> > >         }
> > >         spin_unlock_irqrestore(&eiointc->lock, flags);
> > >
> > > --
> > > 2.39.3
> > >
> >
>