[PATCH v4] kvm: nVMX: Remove superfluous VMX instruction fault checks

[Jim Mattson <jmattson@xxxxxxxxxx>]

According to the Intel SDM, "Certain exceptions have priority over VM
exits. These include invalid-opcode exceptions, faults based on
privilege level*, and general-protection exceptions that are based on
checking I/O permission bits in the task-state segment (TSS)."

There is no need to check for faulting conditions that the hardware
has already checked.

* These include faults generated by attempts to execute, in
  virtual-8086 mode, privileged instructions that are not recognized
  in that mode.

Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
---
 arch/x86/kvm/vmx.c | 55 ++++++++----------------------------------------------
 1 file changed, 8 insertions(+), 47 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 259e9b28ccf8..a6c4af687006 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -7107,34 +7107,14 @@ static int enter_vmx_operation(struct kvm_vcpu *vcpu)
 static int handle_vmon(struct kvm_vcpu *vcpu)
 {
 	int ret;
-	struct kvm_segment cs;
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	const u64 VMXON_NEEDED_FEATURES = FEATURE_CONTROL_LOCKED
 		| FEATURE_CONTROL_VMXON_ENABLED_OUTSIDE_SMX;
 
-	/* The Intel VMX Instruction Reference lists a bunch of bits that
-	 * are prerequisite to running VMXON, most notably cr4.VMXE must be
-	 * set to 1 (see vmx_set_cr4() for when we allow the guest to set this).
-	 * Otherwise, we should fail with #UD. We test these now:
+	/*
+	 * Most of the faulting conditions have already been checked by
+	 * hardware, prior to the VM-exit for VMXON.
 	 */
-	if (!kvm_read_cr4_bits(vcpu, X86_CR4_VMXE) ||
-	    !kvm_read_cr0_bits(vcpu, X86_CR0_PE) ||
-	    (vmx_get_rflags(vcpu) & X86_EFLAGS_VM)) {
-		kvm_queue_exception(vcpu, UD_VECTOR);
-		return 1;
-	}
-
-	vmx_get_segment(vcpu, &cs, VCPU_SREG_CS);
-	if (is_long_mode(vcpu) && !cs.l) {
-		kvm_queue_exception(vcpu, UD_VECTOR);
-		return 1;
-	}
-
-	if (vmx_get_cpl(vcpu)) {
-		kvm_inject_gp(vcpu, 0);
-		return 1;
-	}
-
 	if (vmx->nested.vmxon) {
 		nested_vmx_failValid(vcpu, VMXERR_VMXON_IN_VMX_ROOT_OPERATION);
 		return kvm_skip_emulated_instruction(vcpu);
@@ -7161,29 +7141,15 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
  * Intel's VMX Instruction Reference specifies a common set of prerequisites
  * for running VMX instructions (except VMXON, whose prerequisites are
  * slightly different). It also specifies what exception to inject otherwise.
+ * Note that many of these exceptions have priority over VM exits, so they
+ * don't have to be checked again here.
  */
 static int nested_vmx_check_permission(struct kvm_vcpu *vcpu)
 {
-	struct kvm_segment cs;
-	struct vcpu_vmx *vmx = to_vmx(vcpu);
-
-	if (!vmx->nested.vmxon) {
-		kvm_queue_exception(vcpu, UD_VECTOR);
-		return 0;
-	}
-
-	vmx_get_segment(vcpu, &cs, VCPU_SREG_CS);
-	if ((vmx_get_rflags(vcpu) & X86_EFLAGS_VM) ||
-	    (is_long_mode(vcpu) && !cs.l)) {
+	if (!to_vmx(vcpu)->nested.vmxon) {
 		kvm_queue_exception(vcpu, UD_VECTOR);
 		return 0;
 	}
-
-	if (vmx_get_cpl(vcpu)) {
-		kvm_inject_gp(vcpu, 0);
-		return 0;
-	}
-
 	return 1;
 }
 
@@ -7527,7 +7493,7 @@ static int handle_vmread(struct kvm_vcpu *vcpu)
 		if (get_vmx_mem_address(vcpu, exit_qualification,
 				vmx_instruction_info, true, &gva))
 			return 1;
-		/* _system ok, as nested_vmx_check_permission verified cpl=0 */
+		/* _system ok, as hardware has verified cpl=0 */
 		kvm_write_guest_virt_system(&vcpu->arch.emulate_ctxt, gva,
 			     &field_value, (is_long_mode(vcpu) ? 8 : 4), NULL);
 	}
@@ -7660,7 +7626,7 @@ static int handle_vmptrst(struct kvm_vcpu *vcpu)
 	if (get_vmx_mem_address(vcpu, exit_qualification,
 			vmx_instruction_info, true, &vmcs_gva))
 		return 1;
-	/* ok to use *_system, as nested_vmx_check_permission verified cpl=0 */
+	/* ok to use *_system, as hardware has verified cpl=0 */
 	if (kvm_write_guest_virt_system(&vcpu->arch.emulate_ctxt, vmcs_gva,
 				 (void *)&to_vmx(vcpu)->nested.current_vmptr,
 				 sizeof(u64), &e)) {
@@ -7693,11 +7659,6 @@ static int handle_invept(struct kvm_vcpu *vcpu)
 	if (!nested_vmx_check_permission(vcpu))
 		return 1;
 
-	if (!kvm_read_cr0_bits(vcpu, X86_CR0_PE)) {
-		kvm_queue_exception(vcpu, UD_VECTOR);
-		return 1;
-	}
-
 	vmx_instruction_info = vmcs_read32(VMX_INSTRUCTION_INFO);
 	type = kvm_register_readl(vcpu, (vmx_instruction_info >> 28) & 0xf);
 
-- 
2.13.0.rc0.306.g87b477812d-goog