linux-kernel - [PATCH 3/7] KVM: MMU: document write_flooding

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Date:	Wed, 19 Jun 2013 17:09:21 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	gleb@...hat.com
Cc:	avi.kivity@...il.com, mtosatti@...hat.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Subject: [PATCH 3/7] KVM: MMU: document write_flooding_count

Document write_flooding_count to Documentation/virtual/kvm/mmu.txt

Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
---
 Documentation/virtual/kvm/mmu.txt | 8 ++++++++
 arch/x86/include/asm/kvm_host.h   | 5 +++++
 2 files changed, 13 insertions(+)

diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
index ce6df51..5a6b2e2 100644
--- a/Documentation/virtual/kvm/mmu.txt
+++ b/Documentation/virtual/kvm/mmu.txt
@@ -214,6 +214,14 @@ Shadow pages contain the following information:
     It is only used on 32bit host which helps us to detect whether updating the
     64bit spte is complete so that we can avoid reading the truncated value out
     of mmu-lock.
+  write_flooding_count:
+    In order to sync the page table between guest and host, the page sometimes
+    needs to be write-protected (see "Synchronized and unsynchronized pages"
+    below), any write to the page can cause write emulation. If the emulation
+    on the page is too frequent we'd better unmap the page to avoid the
+    future emulation. write_flooding_count aims at this optimization which is
+    increased when the page needs to be write emulated and cleared when the
+    page is actually used.
 
 Reverse map
 ===========
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 1dac2c1..5eb5382 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -234,6 +234,11 @@ struct kvm_mmu_page {
 	int clear_spte_count;
 #endif
 
+	/*
+	 * Increased when the page needs to be write emulated, cleared
+	 * when the page is actually used as page table to avoid frequent
+	 * emulation on the page.
+	 */
 	int write_flooding_count;
 };
 
-- 
1.8.1.4

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