linux-kernel - Re: [PATCH V3] kvm: check tlbs

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Date:   Mon, 21 Dec 2020 19:31:51 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>, stable@...r.kernel.org
Subject: Re: [PATCH V3] kvm: check tlbs_dirty directly

On 17/12/20 16:41, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> In kvm_mmu_notifier_invalidate_range_start(), tlbs_dirty is used as:
>          need_tlb_flush |= kvm->tlbs_dirty;
> with need_tlb_flush's type being int and tlbs_dirty's type being long.
> 
> It means that tlbs_dirty is always used as int and the higher 32 bits
> is useless.  We need to check tlbs_dirty in a correct way and this
> change checks it directly without propagating it to need_tlb_flush.
> 
> Note: it's _extremely_ unlikely this neglecting of higher 32 bits can
> cause problems in practice.  It would require encountering tlbs_dirty
> on a 4 billion count boundary, and KVM would need to be using shadow
> paging or be running a nested guest.
> 
> Cc: stable@...r.kernel.org
> Fixes: a4ee1ca4a36e ("KVM: MMU: delay flush all tlbs on sync_page path")
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
> Changed from V1:
>          Update the patch and the changelog as Sean Christopherson suggested.
> 
> Changed from v2:
> 	don't change the type of need_tlb_flush
> 
>   virt/kvm/kvm_main.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 2541a17ff1c4..3083fb53861d 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -482,9 +482,8 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>   	kvm->mmu_notifier_count++;
>   	need_tlb_flush = kvm_unmap_hva_range(kvm, range->start, range->end,
>   					     range->flags);
> -	need_tlb_flush |= kvm->tlbs_dirty;
>   	/* we've to flush the tlb before the pages can be freed */
> -	if (need_tlb_flush)
> +	if (need_tlb_flush || kvm->tlbs_dirty)
>   		kvm_flush_remote_tlbs(kvm);
>   
>   	spin_unlock(&kvm->mmu_lock);
> 

Queued, thanks.

Paolo