KVM: selftests: Restore assert for non-nested VMs in access tracking test

Restore the assert (on x86-64) that <10% of pages are still idle when NOT
running as a nested VM in the access tracking test.  The original assert
was converted to a "warning" to avoid false failures when running the
test in a VM, but the non-nested case does not suffer from the same
"infinite TLB size" issue.

Using the HYPERVISOR flag isn't infallible as VMMs aren't strictly
required to enumerate the "feature" in CPUID, but practically speaking
anyone that is running KVM selftests in VMs is going to be using a VMM
and hypervisor that sets the HYPERVISOR flag.

Cc: David Matlack <dmatlack@google.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221129175300.4052283-3-seanjc@google.com

tools/testing/selftests/kvm/access_tracking_perf_test.c

@@ -46,6 +46,7 @@
 #include "test_util.h"
 #include "memstress.h"
 #include "guest_modes.h"
+#include "processor.h"
 
 /* Global variable used to synchronize all of the vCPU threads. */
 static int iteration;
@@ -180,15 +181,21 @@ static void mark_vcpu_memory_idle(struct kvm_vm *vm,
 	 * access tracking but low enough as to not make the test too brittle
 	 * over time and across architectures.
 	 *
-	 * Note that when run in nested virtualization, this check will trigger
-	 * much more frequently because TLB size is unlimited and since no flush
-	 * happens, much more pages are cached there and guest won't see the
-	 * "idle" bit cleared.
+	 * When running the guest as a nested VM, "warn" instead of asserting
+	 * as the TLB size is effectively unlimited and the KVM doesn't
+	 * explicitly flush the TLB when aging SPTEs.  As a result, more pages
+	 * are cached and the guest won't see the "idle" bit cleared.
 	 */
-	if (still_idle >= pages / 10)
+	if (still_idle >= pages / 10) {
+#ifdef __x86_64__
+		TEST_ASSERT(this_cpu_has(X86_FEATURE_HYPERVISOR),
+			    "vCPU%d: Too many pages still idle (%lu out of %lu)",
+			    vcpu_idx, still_idle, pages);
+#endif
 		printf("WARNING: vCPU%d: Too many pages still idle (%lu out of %lu), "
 		       "this will affect performance results.\n",
 		       vcpu_idx, still_idle, pages);
+	}
 
 	close(page_idle_fd);
 	close(pagemap_fd);

tools/testing/selftests/kvm/include/x86_64/processor.h

@@ -94,6 +94,7 @@ struct kvm_x86_cpu_feature {
 #define	X86_FEATURE_XSAVE		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 26)
 #define	X86_FEATURE_OSXSAVE		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 27)
 #define	X86_FEATURE_RDRAND		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 30)
+#define	X86_FEATURE_HYPERVISOR		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 31)
 #define X86_FEATURE_PAE			KVM_X86_CPU_FEATURE(0x1, 0, EDX, 6)
 #define	X86_FEATURE_MCE			KVM_X86_CPU_FEATURE(0x1, 0, EDX, 7)
 #define	X86_FEATURE_APIC		KVM_X86_CPU_FEATURE(0x1, 0, EDX, 9)