linux-kernel - [PATCH 1/7] KVM: Remove redundant smp_mb() in the kvm_mmu_commit_zap

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Message-Id: <1457838631-26775-2-git-send-email-tianyu.lan@intel.com>
Date:	Sun, 13 Mar 2016 11:10:24 +0800
From:	Lan Tianyu <tianyu.lan@...el.com>
To:	gleb@...nel.org, pbonzini@...hat.com, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	guangrong.xiao@...ux.intel.com
Cc:	Lan Tianyu <tianyu.lan@...el.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/7] KVM: Remove redundant smp_mb() in the kvm_mmu_commit_zap_page()

There is already a barrier inside of kvm_flush_remote_tlbs() which can
help to make sure everyone sees our modifications to the page tables and
see changes to vcpu->mode here. So remove the smp_mb in the
kvm_mmu_commit_zap_page() and update the comment.

Signed-off-by: Lan Tianyu <tianyu.lan@...el.com>
---
 arch/x86/kvm/mmu.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 2463de0..5e795af 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2390,14 +2390,12 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 		return;
 
 	/*
-	 * wmb: make sure everyone sees our modifications to the page tables
-	 * rmb: make sure we see changes to vcpu->mode
-	 */
-	smp_mb();
-
-	/*
-	 * Wait for all vcpus to exit guest mode and/or lockless shadow
-	 * page table walks.
+	 * We need to make sure everyone sees our modifications to
+	 * the page tables and see changes to vcpu->mode here. The barrier
+	 * in the kvm_flush_remote_tlbs() helps us to achieve these. This pairs
+	 * with vcpu_enter_guest and walk_shadow_page_lockless_begin/end.
+	 * In addition, wait for all vcpus to exit guest mode and/or lockless
+	 * shadow page table walks.
 	 */
 	kvm_flush_remote_tlbs(kvm);
 
-- 
1.8.4.rc0.1.g8f6a3e5.dirty