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

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Message-ID: <6068ef0b-bf04-03f7-b867-02d8a450d392@loongson.cn>
Date: Fri, 27 Jun 2025 09:58:49 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>,
 David Laight <david.laight.linux@...il.com>
Cc: 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



On 2025/6/21 下午9:04, Huacai Chen wrote:
> 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.
By my understanding, the default case cannot happen never with iocsr 
emulation function kvm_emu_iocsr() in kernel in the previous patch.

David suggests that default case can be removed since it will not 
happen. It actually makes code quicker if compared with assembly 
language, since there is one if-else comparison reduction.

Regards
Bibo Mao

> 
> 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
>>>>
>>>
>>