[PATCH] KVM: SVM: Connect 'npt' module param to KVM's internal 'npt_enabled'

[Sean Christopherson <seanjc@google.com>]

Directly connect the 'npt' param to the 'npt_enabled' variable so that
runtime adjustments to npt_enabled are reflected in sysfs.  Move the
!PAE restriction to a runtime check to ensure NPT is forced off if the
host is using 2-level paging, and add a comment explicitly stating why
NPT requires a 64-bit kernel or a kernel with PAE enabled.

Opportunistically switch the param to octal permissions.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/svm.c | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 54610270f66a..0ee74321461e 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -115,13 +115,6 @@ static const struct svm_direct_access_msrs {
 	{ .index = MSR_INVALID,				.always = false },
 };
 
-/* enable NPT for AMD64 and X86 with PAE */
-#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
-bool npt_enabled = true;
-#else
-bool npt_enabled;
-#endif
-
 /*
  * These 2 parameters are used to config the controls for Pause-Loop Exiting:
  * pause_filter_count: On processors that support Pause filtering(indicated
@@ -170,9 +163,12 @@ module_param(pause_filter_count_shrink, ushort, 0444);
 static unsigned short pause_filter_count_max = KVM_SVM_DEFAULT_PLE_WINDOW_MAX;
 module_param(pause_filter_count_max, ushort, 0444);
 
-/* allow nested paging (virtualized MMU) for all guests */
-static int npt = true;
-module_param(npt, int, S_IRUGO);
+/*
+ * Use nested page tables by default.  Note, NPT may get forced off by
+ * svm_hardware_setup() if it's unsupported by hardware or the host kernel.
+ */
+bool npt_enabled = true;
+module_param_named(npt, npt_enabled, bool, 0444);
 
 /* allow nested virtualization in KVM/SVM */
 static int nested = true;
@@ -988,12 +984,17 @@ static __init int svm_hardware_setup(void)
 			goto err;
 	}
 
+	/*
+	 * KVM's MMU doesn't support using 2-level paging for itself, and thus
+	 * NPT isn't supported if the host is using 2-level paging since host
+	 * CR4 is unchanged on VMRUN.
+	 */
+	if (!IS_ENABLED(CONFIG_X86_64) && !IS_ENABLED(CONFIG_X86_PAE))
+		npt_enabled = false;
+
 	if (!boot_cpu_has(X86_FEATURE_NPT))
 		npt_enabled = false;
 
-	if (npt_enabled && !npt)
-		npt_enabled = false;
-
 	kvm_configure_mmu(npt_enabled, get_max_npt_level(), PG_LEVEL_1G);
 	pr_info("kvm: Nested Paging %sabled\n", npt_enabled ? "en" : "dis");
 
-- 
2.30.1.766.gb4fecdf3b7-goog