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

From: Luca Tettamanti
Date: Tue Jun 19 2007 - 16:41:56 EST


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@xxxxxxxxx>

---
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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