KVM: Remove duplicated zero clear with dirty_bitmap buffer

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

Posted by Bibo Mao 1 year, 8 months ago

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@loongson.cn>
---
 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

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

Posted by Christophe JAILLET 1 year, 8 months ago

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@loongson.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.

CJ

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

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

Posted by maobibo 1 year, 8 months ago


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@loongson.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

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

Posted by Paolo Bonzini 1 year, 7 months ago

On 6/14/24 04:45, maobibo wrote:
> 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?

The advantage is that you get (a lot) fewer vmexits to set the dirty 
bitmap, and that write protection is not done in a single expensive 
step.  Instead it is done at the time that userspace first clears the 
bits in the dirty bitmap.  It provides much better performance.

Paolo

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

Posted by maobibo 1 year, 7 months ago


On 2024/6/21 上午5:17, Paolo Bonzini wrote:
> On 6/14/24 04:45, maobibo wrote:
>> 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?
> 
> The advantage is that you get (a lot) fewer vmexits to set the dirty 
> bitmap, and that write protection is not done in a single expensive 
> step.  Instead it is done at the time that userspace first clears the 
> bits in the dirty bitmap.  It provides much better performance.
Got it, thanks for the explanation .

Regards
Bibo Mao
> 
> Paolo
>