KVM: VMX: Fix and tweak the comments for VM-Enter

Fix an incorrect/stale comment regarding the vmx_vcpu pointer, as guest
registers are now loaded using a direct pointer to the start of the
register array.

Opportunistically add a comment to document why the vmx_vcpu pointer is
needed, its consumption via 'call vmx_update_host_rsp' is rather subtle.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmenter.S

@@ -94,7 +94,7 @@ ENDPROC(vmx_vmexit)
 
 /**
  * __vmx_vcpu_run - Run a vCPU via a transition to VMX guest mode
- * @vmx:	struct vcpu_vmx *
+ * @vmx:	struct vcpu_vmx * (forwarded to vmx_update_host_rsp)
  * @regs:	unsigned long * (to guest registers)
  * @launched:	%true if the VMCS has been launched
  *
@@ -151,7 +151,7 @@ ENTRY(__vmx_vcpu_run)
 	mov VCPU_R14(%_ASM_AX), %r14
 	mov VCPU_R15(%_ASM_AX), %r15
 #endif
-	/* Load guest RAX.  This kills the vmx_vcpu pointer! */
+	/* Load guest RAX.  This kills the @regs pointer! */
 	mov VCPU_RAX(%_ASM_AX), %_ASM_AX
 
 	/* Enter guest mode */