[PATCH] KVM: vmx: speed up MSR bitmap merge

[Paolo Bonzini]

The bulk of the MSR bitmap is either immutable, or can be copied from
the L1 bitmap.  By initializing it at VMXON time, and copying the mutable
parts one long at a time on vmentry (rather than one bit), about 4000
clock cycles (30%) can be saved on a nested VMLAUNCH/VMRESUME.

The resulting for loop only has four iterations, so it is cheap enough
to reinitialize the MSR write bitmaps on every iteration, and it makes
the code simpler.

Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/vmx.c | 57 ++++++++++++++++++++++++++++--------------------------
 1 file changed, 30 insertions(+), 27 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 1458cb52de68..ee214b4112af 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -5217,11 +5217,6 @@ static void nested_vmx_disable_intercept_for_msr(unsigned long *msr_bitmap_l1,
 {
 	int f = sizeof(unsigned long);
 
-	if (!cpu_has_vmx_msr_bitmap()) {
-		WARN_ON(1);
-		return;
-	}
-
 	/*
 	 * See Intel PRM Vol. 3, 20.6.9 (MSR-Bitmap Address). Early manuals
 	 * have the write-low and read-high bitmap offsets the wrong way round.
@@ -7493,6 +7488,7 @@ static int enter_vmx_operation(struct kvm_vcpu *vcpu)
 				(unsigned long *)__get_free_page(GFP_KERNEL);
 		if (!vmx->nested.msr_bitmap)
 			goto out_msr_bitmap;
+		memset(vmx->nested.msr_bitmap, 0xff, PAGE_SIZE);
 	}
 
 	vmx->nested.cached_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL);
@@ -10325,36 +10321,43 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
 	/* This shortcut is ok because we support only x2APIC MSRs so far. */
 	if (!nested_cpu_has_virt_x2apic_mode(vmcs12))
 		return false;
+	if (WARN_ON_ONCE(!cpu_has_vmx_msr_bitmap()))
+		return false;
 
 	page = kvm_vcpu_gpa_to_page(vcpu, vmcs12->msr_bitmap);
 	if (is_error_page(page))
 		return false;
-	msr_bitmap_l1 = (unsigned long *)kmap(page);
 
-	memset(msr_bitmap_l0, 0xff, PAGE_SIZE);
+	msr_bitmap_l1 = (unsigned long *)kmap(page);
+	if (nested_cpu_has_apic_reg_virt(vmcs12)) {
+		/* Disable read intercept for all MSRs between 0x800 and 0x8ff.  */
+		for (msr = 0x800; msr <= 0x8ff; msr += BITS_PER_LONG) {
+			unsigned word = msr / BITS_PER_LONG;
+			msr_bitmap_l0[word] = msr_bitmap_l1[word];
+			msr_bitmap_l0[word + (0x800 / sizeof(long))] = ~0;
+		}
+	} else {
+		for (msr = 0x800; msr <= 0x8ff; msr += BITS_PER_LONG) {
+			unsigned word = msr / BITS_PER_LONG;
+			msr_bitmap_l0[word] = ~0;
+			msr_bitmap_l0[word + (0x800 / sizeof(long))] = ~0;
+		}
+	}
 
-	if (nested_cpu_has_virt_x2apic_mode(vmcs12)) {
-		if (nested_cpu_has_apic_reg_virt(vmcs12))
-			for (msr = 0x800; msr <= 0x8ff; msr++)
-				nested_vmx_disable_intercept_for_msr(
-					msr_bitmap_l1, msr_bitmap_l0,
-					msr, MSR_TYPE_R);
+	nested_vmx_disable_intercept_for_msr(
+		msr_bitmap_l1, msr_bitmap_l0,
+		APIC_BASE_MSR + (APIC_TASKPRI >> 4),
+		MSR_TYPE_W);
 
+	if (nested_cpu_has_vid(vmcs12)) {
 		nested_vmx_disable_intercept_for_msr(
-				msr_bitmap_l1, msr_bitmap_l0,
-				APIC_BASE_MSR + (APIC_TASKPRI >> 4),
-				MSR_TYPE_R | MSR_TYPE_W);
-
-		if (nested_cpu_has_vid(vmcs12)) {
-			nested_vmx_disable_intercept_for_msr(
-				msr_bitmap_l1, msr_bitmap_l0,
-				APIC_BASE_MSR + (APIC_EOI >> 4),
-				MSR_TYPE_W);
-			nested_vmx_disable_intercept_for_msr(
-				msr_bitmap_l1, msr_bitmap_l0,
-				APIC_BASE_MSR + (APIC_SELF_IPI >> 4),
-				MSR_TYPE_W);
-		}
+			msr_bitmap_l1, msr_bitmap_l0,
+			APIC_BASE_MSR + (APIC_EOI >> 4),
+			MSR_TYPE_W);
+		nested_vmx_disable_intercept_for_msr(
+			msr_bitmap_l1, msr_bitmap_l0,
+			APIC_BASE_MSR + (APIC_SELF_IPI >> 4),
+			MSR_TYPE_W);
 	}
 	kunmap(page);
 	kvm_release_page_clean(page);
-- 
1.8.3.1