linux-kernel - Re: [PATCH] KVM: Remove duplicated zero clear with dirty

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Date: Fri, 14 Jun 2024 10:45:56 +0800
From: maobibo <maobibo@...ngson.cn>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Remove duplicated zero clear with dirty_bitmap
 buffer



On 2024/6/14 上午3:25, Christophe JAILLET wrote:
> Le 13/06/2024 à 14:54, Bibo Mao a écrit :
>> Since dirty_bitmap pointer is allocated with function __vcalloc(),
>> there is __GFP_ZERO flag set in the implementation about this function
>> __vcalloc_noprof(). It is not necessary to clear dirty_bitmap buffer
>> with zero again.
>>
>> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
>> ---
>>   virt/kvm/kvm_main.c | 3 ---
>>   1 file changed, 3 deletions(-)
>>
> 
> Hi,
> 
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 14841acb8b95..c7d4a041dcfa 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -1669,9 +1669,6 @@ static int kvm_prepare_memory_region(struct kvm 
>> *kvm,
>>               r = kvm_alloc_dirty_bitmap(new);
>>               if (r)
>>                   return r;
>> -
>> -            if (kvm_dirty_log_manual_protect_and_init_set(kvm))
>> -                bitmap_set(new->dirty_bitmap, 0, new->npages);
> 
> unless I miss something obvious, this does not clear anything, but set 
> all bits to 1.
> 
> 0 is not for "write 0" (i.e. clear), but for "start at offset 0".
> So all bits are set to 1 in this case.
you are right, I had thought it is to write 0 :(

I do not know whether KVM_DIRTY_LOG_INITIALLY_SET should be enabled on 
LoongArch. If it is set, write protection for second MMU will start one 
by one in function kvm_arch_mmu_enable_log_dirty_pt_masked() when dirty 
log is cleared if it is set, else write protection will start in 
function kvm_arch_commit_memory_region() when flag of memslot is changed.

I do not see the obvious benefits between these two write protect 
stages. Can anyone give me any hints?

Regards
Bibo Mao

> 
> CJ
> 
>>           }
>>       }
>>
>> base-commit: 83a7eefedc9b56fe7bfeff13b6c7356688ffa670