2024-06-13 12:54:22

by Bibo Mao

Subject: [PATCH] KVM: Remove duplicated zero clear with dirty_bitmap buffer

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 <[email protected]>
---
virt/kvm/kvm_main.c | 3 ---
1 file changed, 3 deletions(-)

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);
}
}


base-commit: 83a7eefedc9b56fe7bfeff13b6c7356688ffa670
--
2.39.3

2024-06-13 19:48:08

by Christophe JAILLET

Subject: Re: [PATCH] KVM: Remove duplicated zero clear with dirty_bitmap buffer

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 <[email protected]>
> ---
> 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.

CJ

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

2024-06-14 02:53:56

by Bibo Mao

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 <[email protected]>
>> ---
>>   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