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

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Date: Thu, 13 Jun 2024 21:25:38 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Bibo Mao <maobibo@...ngson.cn>, 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

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.

CJ

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