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

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Date:   Thu, 17 Dec 2020 23:41:18 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>, stable@...r.kernel.org
Subject: [PATCH V3] kvm: check tlbs_dirty directly

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);
-- 
2.19.1.6.gb485710b