[PATCH v2 12/12] selftests: KVM: Add counter emulation benchmark

Oliver Upton
2021-07-16T21:27:00Z

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 <ricarkol@google.com>
Signed-off-by: Oliver Upton <oupton@google.com>
---
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");
+ sync_global_from_guest(vm, counter_values);
+
+ if (csv)
+ log_csv(csv, trapped);
+
+ delta = counter_values.cntvct_end - counter_values.cntvct_start;
+ avg = ((avg * i) + delta) / (i + 1);
+ }
+
+ return avg;
+}
+
+static void setup_counter(struct kvm_vm *vm, uint64_t offset)
+{
+ vcpu_access_device_attr(vm, VCPU_ID, KVM_ARM_VCPU_TIMER_CTRL,
+ KVM_ARM_VCPU_TIMER_OFFSET_PTIMER, &offset,
+ true);
+}
+
+static void run_tests(struct kvm_vm *vm, FILE *csv)
+{
+ double avg_trapped, avg_native, freq;
+
+ freq = counter_frequency();
+
+ if (csv)
+ fputs("trapped,freq_mhz,cntvct_start,cntpct,cntvct_end\n", csv);
+
+ /* no physical offsetting; kvm allows reads of cntpct_el0 */
+ setup_counter(vm, 0);
+ avg_native = run_loop(vm, csv, false);
+
+ /* force emulation of the physical counter */
+ setup_counter(vm, 1);
+ avg_trapped = run_loop(vm, csv, true);
+
+ TEST_ASSERT(enter_guest(vm), "guest didn't run to completion");
+ pr_info("%lu iterations: average cycles (@%.02fMHz) native: %.02f, trapped: %.02f\n",
+ nr_iterations, freq, avg_native, avg_trapped);
+}
+
+static void usage(const char *program_name)
+{
+ fprintf(stderr,
+ "Usage: %s [-h] [-o csv_file] [-n iterations]\n"
+ " -h prints this message\n"
+ " -n number of test iterations (default: %lu)\n"
+ " -o csv file to write data\n",
+ program_name, nr_iterations);
+}
+
+int main(int argc, char **argv)
+{
+ struct kvm_vm *vm;
+ FILE *csv = NULL;
+ int opt;
+
+ while ((opt = getopt(argc, argv, "hn:o:")) != -1) {
+ switch (opt) {
+ case 'o':
+ csv = fopen(optarg, "w");
+ if (!csv) {
+ fprintf(stderr, "failed to open file '%s': %d\n",
+ optarg, errno);
+ exit(1);
+ }
+ break;
+ case 'n':
+ nr_iterations = strtoul(optarg, NULL, 0);
+ break;
+ default:
+ fprintf(stderr, "unrecognized option: '-%c'\n", opt);
+ /* fallthrough */
+ case 'h':
+ usage(argv[0]);
+ exit(1);
+ }
+ }
+
+ vm = vm_create_default(VCPU_ID, 0, guest_main);
+ sync_global_to_guest(vm, nr_iterations);
+ ucall_init(vm, NULL);
+
+ if (_vcpu_has_device_attr(vm, VCPU_ID, KVM_ARM_VCPU_TIMER_CTRL,
+ KVM_ARM_VCPU_TIMER_OFFSET_PTIMER)) {
+ print_skip("KVM_ARM_VCPU_TIMER_OFFSET_PTIMER not supported.");
+ exit(KSFT_SKIP);
+ }
+
+ run_tests(vm, csv);
+ kvm_vm_free(vm);
+
+ if (csv)
+ fclose(csv);
+}
--
2.32.0.402.g57bb445576-goog