KVM: x86 emulator: add get_cached_segment_base() callback to x86_emulate_ops

On VMX it is expensive to call get_cached_descriptor() just to get segment
base since multiple vmcs_reads are done instead of only one. Introduce
new call back get_cached_segment_base() for efficiency.
Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

arch/x86/include/asm/kvm_emulate.h

@@ -132,6 +132,7 @@ struct x86_emulate_ops {
 				      int seg, struct kvm_vcpu *vcpu);
 	u16 (*get_segment_selector)(int seg, struct kvm_vcpu *vcpu);
 	void (*set_segment_selector)(u16 sel, int seg, struct kvm_vcpu *vcpu);
+	unsigned long (*get_cached_segment_base)(int seg, struct kvm_vcpu *vcpu);
 	void (*get_gdt)(struct desc_ptr *dt, struct kvm_vcpu *vcpu);
 	ulong (*get_cr)(int cr, struct kvm_vcpu *vcpu);
 	void (*set_cr)(int cr, ulong val, struct kvm_vcpu *vcpu);

arch/x86/kvm/emulate.c

@@ -2097,17 +2097,6 @@ static bool emulator_io_permited(struct x86_emulate_ctxt *ctxt,
 	return true;
 }
 
-static u32 get_cached_descriptor_base(struct x86_emulate_ctxt *ctxt,
-				      struct x86_emulate_ops *ops,
-				      int seg)
-{
-	struct desc_struct desc;
-	if (ops->get_cached_descriptor(&desc, seg, ctxt->vcpu))
-		return get_desc_base(&desc);
-	else
-		return ~0;
-}
-
 static void save_state_to_tss16(struct x86_emulate_ctxt *ctxt,
 				struct x86_emulate_ops *ops,
 				struct tss_segment_16 *tss)
@@ -2383,7 +2372,7 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
 	int ret;
 	u16 old_tss_sel = ops->get_segment_selector(VCPU_SREG_TR, ctxt->vcpu);
 	ulong old_tss_base =
-		get_cached_descriptor_base(ctxt, ops, VCPU_SREG_TR);
+		ops->get_cached_segment_base(VCPU_SREG_TR, ctxt->vcpu);
 	u32 desc_limit;
 
 	/* FIXME: old_tss_base == ~0 ? */

arch/x86/kvm/x86.c

@@ -3714,6 +3714,12 @@ static void emulator_get_gdt(struct desc_ptr *dt, struct kvm_vcpu *vcpu)
 	kvm_x86_ops->get_gdt(vcpu, dt);
 }
 
+static unsigned long emulator_get_cached_segment_base(int seg,
+						      struct kvm_vcpu *vcpu)
+{
+	return get_segment_base(vcpu, seg);
+}
+
 static bool emulator_get_cached_descriptor(struct desc_struct *desc, int seg,
 					   struct kvm_vcpu *vcpu)
 {
@@ -3804,6 +3810,7 @@ static struct x86_emulate_ops emulate_ops = {
 	.set_cached_descriptor = emulator_set_cached_descriptor,
 	.get_segment_selector = emulator_get_segment_selector,
 	.set_segment_selector = emulator_set_segment_selector,
+	.get_cached_segment_base = emulator_get_cached_segment_base,
 	.get_gdt             = emulator_get_gdt,
 	.get_cr              = emulator_get_cr,
 	.set_cr              = emulator_set_cr,