Re: [PATCH V3] kvm: check tlbs_dirty directly

[Paolo Bonzini]

On 17/12/20 16:41, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx
> Fixes: a4ee1ca4a36e ("KVM: MMU: delay flush all tlbs on sync_page path")
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> ---
> 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