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Subject: Re: [PATCH] KVM: Discard zero mask with function kvm_dirty_ring_reset
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
 linux-kernel@vger.kernel.org
References: <20240613122803.1031511-1-maobibo@loongson.cn>
 <Zmsg8ciwSp1a_864@google.com>
From: maobibo <maobibo@loongson.cn>
Message-ID: <3cf851a2-be51-f227-82a4-090de01bc8be@loongson.cn>
Date: Fri, 14 Jun 2024 10:58:09 +0800
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On 2024/6/14 上午12:43, Sean Christopherson wrote:
> On Thu, Jun 13, 2024, Bibo Mao wrote:
>> Function kvm_reset_dirty_gfn may be called with parameters cur_slot /
>> cur_offset / mask are all zero, it does not represent real dirty page.
>> It is not necessary to clear dirty page in this condition. Also return
>> value of macro __fls() is undefined if mask is zero which is called in
>> funciton kvm_reset_dirty_gfn(). Here just discard it.
>>
>> Signed-off-by: Bibo Mao <maobibo@loongson.cn>
>> ---
>>   virt/kvm/dirty_ring.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
>> index 86d267db87bb..05f4c1c40cc7 100644
>> --- a/virt/kvm/dirty_ring.c
>> +++ b/virt/kvm/dirty_ring.c
>> @@ -147,14 +147,16 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
>>   				continue;
>>   			}
>>   		}
>> -		kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>> +		if (mask)
>> +			kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>>   		cur_slot = next_slot;
>>   		cur_offset = next_offset;
>>   		mask = 1;
>>   		first_round = false;
>>   	}
>>   
>> -	kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>> +	if (mask)
>> +		kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> 
> Given that mask must be checked before __fls(), just do:
That is ok for me. To be frankly I am not familiar with kvm common code,
I submit this patch just when I look through the migration source code.

Regards
Bibo Mao
> 
> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> index 86d267db87bb..7bc74969a819 100644
> --- a/virt/kvm/dirty_ring.c
> +++ b/virt/kvm/dirty_ring.c
> @@ -55,6 +55,9 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
>          struct kvm_memory_slot *memslot;
>          int as_id, id;
>   
> +       if (!mask)
> +               return;
> +
>          as_id = slot >> 16;
>          id = (u16)slot;
>