linux-kernel - [PATCH 2/2] kvm: avoid useless memory write when possible

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Message-ID: <20070619204138.GB18800@dreamland.darkstar.lan>
Date:	Tue, 19 Jun 2007 22:41:38 +0200
From:	Luca Tettamanti <kronos.it@...il.com>
To:	kvm-devel@...ts.sourceforge.net
Cc:	Avi Kivity <avi@...ranet.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] kvm: avoid useless memory write when possible

When writing to normal memory and the memory area is unchanged the write
can be safely skipped, avoiding the costly kvm_mmu_pte_write.

Signed-Off-By: Luca Tettamanti <kronos.it@...il.com>

---
 Tested with Fedora7, Solaris10 and WinXP on a 32 bit host with Intel
 CPU.

 drivers/kvm/kvm_main.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index 633c2ed..9b7b0b9 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -1139,8 +1139,10 @@ static int emulator_write_phys(struct kvm_vcpu *vcpu, gpa_t gpa,
 		return 0;
 	mark_page_dirty(vcpu->kvm, gpa >> PAGE_SHIFT);
 	virt = kmap_atomic(page, KM_USER0);
-	kvm_mmu_pte_write(vcpu, gpa, virt + offset, val, bytes);
-	memcpy(virt + offset_in_page(gpa), val, bytes);
+	if (memcmp(virt + offset_in_page(gpa), val, bytes)) {
+		kvm_mmu_pte_write(vcpu, gpa, virt + offset, val, bytes);
+		memcpy(virt + offset_in_page(gpa), val, bytes);
+	}
 	kunmap_atomic(virt, KM_USER0);
 	return 1;
 }

Luca
-- 
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