Add a test case for counter emulation on arm64. A side effect of how KVM
handles physical counter offsetting on non-ECV systems is that the
virtual counter will always hit hardware and the physical could be
emulated. Force emulation by writing a nonzero offset to the physical
counter and compare the elapsed cycles to a direct read of the hardware
register.

Reviewed-by: Ricardo Koller <redacted>
Signed-off-by: Oliver Upton <redacted>
---
 tools/testing/selftests/kvm/.gitignore        |   1 +
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../kvm/aarch64/counter_emulation_benchmark.c | 215 ++++++++++++++++++
 3 files changed, 217 insertions(+)
 create mode 100644 tools/testing/selftests/kvm/aarch64/counter_emulation_benchmark.c

diff --git a/tools/testing/selftests/kvm/.gitignore b/tools/testing/selftests/kvm/.gitignore
index 2752813d5090..1d811c6a769b 100644
--- a/tools/testing/selftests/kvm/.gitignore
+++ b/tools/testing/selftests/kvm/.gitignore

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 /aarch64/debug-exceptions
+/aarch64/counter_emulation_benchmark

 /aarch64/get-reg-list
 /aarch64/vgic_init
 /s390x/memop

diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index d89908108c97..e560a3e74bc2 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile

@@ -86,6 +86,7 @@ TEST_GEN_PROGS_x86_64 += kvm_binary_stats_test
 TEST_GEN_PROGS_x86_64 += system_counter_offset_test
 
 TEST_GEN_PROGS_aarch64 += aarch64/debug-exceptions
+TEST_GEN_PROGS_aarch64 += aarch64/counter_emulation_benchmark

 TEST_GEN_PROGS_aarch64 += aarch64/get-reg-list
 TEST_GEN_PROGS_aarch64 += aarch64/vgic_init
 TEST_GEN_PROGS_aarch64 += demand_paging_test

diff --git a/tools/testing/selftests/kvm/aarch64/counter_emulation_benchmark.c b/tools/testing/selftests/kvm/aarch64/counter_emulation_benchmark.c
new file mode 100644
index 000000000000..73aeb6cdebfe
--- /dev/null
+++ b/tools/testing/selftests/kvm/aarch64/counter_emulation_benchmark.c

@@ -0,0 +1,215 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * counter_emulation_benchmark.c -- test to measure the effects of counter
+ * emulation on guest reads of the physical counter.
+ *
+ * Copyright (c) 2021, Google LLC.
+ */
+
+#define _GNU_SOURCE
+#include <asm/kvm.h>
+#include <linux/kvm.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "kvm_util.h"
+#include "processor.h"
+#include "test_util.h"
+
+#define VCPU_ID 0
+
+static struct counter_values {
+	uint64_t cntvct_start;
+	uint64_t cntpct;
+	uint64_t cntvct_end;
+} counter_values;
+
+static uint64_t nr_iterations = 1000;
+
+static void do_test(void)
+{
+	/*
+	 * Open-coded approach instead of using helper methods to keep a tight
+	 * interval around the physical counter read.
+	 */
+	asm volatile("isb\n\t"
+		     "mrs %[cntvct_start], cntvct_el0\n\t"
+		     "isb\n\t"
+		     "mrs %[cntpct], cntpct_el0\n\t"
+		     "isb\n\t"
+		     "mrs %[cntvct_end], cntvct_el0\n\t"
+		     "isb\n\t"
+		     : [cntvct_start] "=r"(counter_values.cntvct_start),
+		     [cntpct] "=r"(counter_values.cntpct),
+		     [cntvct_end] "=r"(counter_values.cntvct_end));
+}
+
+static void guest_main(void)
+{
+	int i;
+
+	for (i = 0; i < nr_iterations; i++) {
+		do_test();
+		GUEST_SYNC(i);
+	}
+
+	for (i = 0; i < nr_iterations; i++) {
+		do_test();
+		GUEST_SYNC(i);
+	}
+
+	GUEST_DONE();
+}
+
+static bool enter_guest(struct kvm_vm *vm)
+{
+	struct ucall uc;
+
+	vcpu_ioctl(vm, VCPU_ID, KVM_RUN, NULL);
+
+	switch (get_ucall(vm, VCPU_ID, &uc)) {
+	case UCALL_DONE:
+		return true;
+	case UCALL_SYNC:
+		break;
+	case UCALL_ABORT:
+		TEST_ASSERT(false, "%s at %s:%ld", (const char *)uc.args[0],
+			    __FILE__, uc.args[1]);
+		break;
+	default:
+		TEST_ASSERT(false, "unexpected exit: %s",
+			    exit_reason_str(vcpu_state(vm, VCPU_ID)->exit_reason));
+		break;
+	}
+
+	/* more work to do in the guest */
+	return false;
+}
+
+static double counter_frequency(void)
+{
+	uint32_t freq;
+
+	asm volatile("mrs %0, cntfrq_el0"
+		     : "=r" (freq));
+
+	return freq / 1000000.0;
+}
+
+static void log_csv(FILE *csv, bool trapped)
+{
+	double freq = counter_frequency();
+
+	fprintf(csv, "%s,%.02f,%lu,%lu,%lu\n",
+		trapped ? "true" : "false", freq,
+		counter_values.cntvct_start,
+		counter_values.cntpct,
+		counter_values.cntvct_end);
+}
+
+static double run_loop(struct kvm_vm *vm, FILE *csv, bool trapped)
+{
+	double avg = 0;
+	int i;
+
+	for (i = 0; i < nr_iterations; i++) {
+		uint64_t delta;
+
+		TEST_ASSERT(!enter_guest(vm), "guest exited unexpectedly");