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: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

3 files changed, 9 insertions, 12 deletions

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index f751657be732..df53ba2294b6 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/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);
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 7c8ed560fd41..8228778ace38 100644
--- a/arch/x86/kvm/emulate.c
+++ b/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 ? */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e3a5455049b0..9a469df6011c 100644
--- a/arch/x86/kvm/x86.c
+++ b/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,