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

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Message-ID: <5b9ed28a-8c25-520b-0663-844c7014d078@loongson.cn>
Date: Fri, 27 Jun 2025 10:12:19 +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

That is a little different with x86/arm64 kvm.
With iocsr address space, there is access size with 1/2/4/8. With IO 
memory address space, there is generic access size with 1/2/4/8, however 
if irqchip driver of VM is deployed in user space, it will be possible 
with 16/32 bytes with SIMD instruction.

Regards
Bibo Mao

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.
> 
> 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
>>>>
>>>
>>