linux-kernel - Re: [PATCH] KVM: Discard zero mask with function kvm_dirty_ring

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Date: Thu, 13 Jun 2024 09:43:38 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Bibo Mao <maobibo@...ngson.cn>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Discard zero mask with function kvm_dirty_ring_reset

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@...ngson.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:

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;