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// SPDX-License-Identifier: GPL-2.0
* access_tracking_perf_test
* Copyright (C) 2021, Google, Inc.
* This test measures the performance effects of KVM's access tracking.
* Access tracking is driven by the MMU notifiers test_young, clear_young, and
* clear_flush_young. These notifiers do not have a direct userspace API,
* however the clear_young notifier can be triggered by marking a pages as idle
* in /sys/kernel/mm/page_idle/bitmap. This test leverages that mechanism to
* enable access tracking on guest memory.
* To measure performance this test runs a VM with a configurable number of
* vCPUs that each touch every page in disjoint regions of memory. Performance
* is measured in the time it takes all vCPUs to finish touching their
* predefined region.
* Note that a deterministic correctness test of access tracking is not possible
* by using page_idle as it exists today. This is for a few reasons:
* 1. page_idle only issues clear_young notifiers, which lack a TLB flush. This
* means subsequent guest accesses are not guaranteed to see page table
* updates made by KVM until some time in the future.
* 2. page_idle only operates on LRU pages. Newly allocated pages are not
* immediately allocated to LRU lists. Instead they are held in a "pagevec",
* which is drained to LRU lists some time in the future. There is no
* userspace API to force this drain to occur.
* These limitations are worked around in this test by using a large enough
* region of memory for each vCPU such that the number of translations cached in
* the TLB and the number of pages held in pagevecs are a small fraction of the
* overall workload. And if either of those conditions are not true this test
* will fail rather than silently passing.
#include <inttypes.h>
#include <limits.h>
#include <pthread.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include "kvm_util.h"
#include "test_util.h"
#include "perf_test_util.h"
#include "guest_modes.h"
/* Global variable used to synchronize all of the vCPU threads. */
static int iteration;
/* Defines what vCPU threads should do during a given iteration. */
static enum {
/* Run the vCPU to access all its memory. */
/* Mark the vCPU's memory idle in page_idle. */
} iteration_work;
/* Set to true when vCPU threads should exit. */
static bool done;
/* The iteration that was last completed by each vCPU. */
static int vcpu_last_completed_iteration[KVM_MAX_VCPUS];
/* Whether to overlap the regions of memory vCPUs access. */
static bool overlap_memory_access;
struct test_params {
/* The backing source for the region of memory. */
enum vm_mem_backing_src_type backing_src;
/* The amount of memory to allocate for each vCPU. */
uint64_t vcpu_memory_bytes;
/* The number of vCPUs to create in the VM. */
int vcpus;
static uint64_t pread_uint64(int fd, const char *filename, uint64_t index)
uint64_t value;
off_t offset = index * sizeof(value);
TEST_ASSERT(pread(fd, &value, sizeof(value), offset) == sizeof(value),
"pread from %s offset 0x%" PRIx64 " failed!",
filename, offset);
return value;
#define PAGEMAP_PRESENT (1ULL << 63)
#define PAGEMAP_PFN_MASK ((1ULL << 55) - 1)
static uint64_t lookup_pfn(int pagemap_fd, struct kvm_vm *vm, uint64_t gva)
uint64_t hva = (uint64_t) addr_gva2hva(vm, gva);
uint64_t entry;
uint64_t pfn;
entry = pread_uint64(pagemap_fd, "pagemap", hva / getpagesize());
if (!(entry & PAGEMAP_PRESENT))
return 0;
pfn = entry & PAGEMAP_PFN_MASK;
if (!pfn) {
print_skip("Looking up PFNs requires CAP_SYS_ADMIN");
return pfn;
static bool is_page_idle(int page_idle_fd, uint64_t pfn)
uint64_t bits = pread_uint64(page_idle_fd, "page_idle", pfn / 64);
return !!((bits >> (pfn % 64)) & 1);
static void mark_page_idle(int page_idle_fd, uint64_t pfn)
uint64_t bits = 1ULL << (pfn % 64);
TEST_ASSERT(pwrite(page_idle_fd, &bits, 8, 8 * (pfn / 64)) == 8,
"Set page_idle bits for PFN 0x%" PRIx64, pfn);
static void mark_vcpu_memory_idle(struct kvm_vm *vm, int vcpu_id)
uint64_t base_gva = perf_test_args.vcpu_args[vcpu_id].gva;
uint64_t pages = perf_test_args.vcpu_args[vcpu_id].pages;
uint64_t page;
uint64_t still_idle = 0;
uint64_t no_pfn = 0;
int page_idle_fd;
int pagemap_fd;
/* If vCPUs are using an overlapping region, let vCPU 0 mark it idle. */
if (overlap_memory_access && vcpu_id)
page_idle_fd = open("/sys/kernel/mm/page_idle/bitmap", O_RDWR);
TEST_ASSERT(page_idle_fd > 0, "Failed to open page_idle.");
pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
TEST_ASSERT(pagemap_fd > 0, "Failed to open pagemap.");
for (page = 0; page < pages; page++) {
uint64_t gva = base_gva + page * perf_test_args.guest_page_size;
uint64_t pfn = lookup_pfn(pagemap_fd, vm, gva);
if (!pfn) {
if (is_page_idle(page_idle_fd, pfn)) {
mark_page_idle(page_idle_fd, pfn);
* Assumption: Less than 1% of pages are going to be swapped out from
* under us during this test.
TEST_ASSERT(no_pfn < pages / 100,
"vCPU %d: No PFN for %" PRIu64 " out of %" PRIu64 " pages.",
vcpu_id, no_pfn, pages);
* Test that at least 90% of memory has been marked idle (the rest might
* not be marked idle because the pages have not yet made it to an LRU
* list or the translations are still cached in the TLB). 90% is
* arbitrary; high enough that we ensure most memory access went through
* access tracking but low enough as to not make the test too brittle
* over time and across architectures.
TEST_ASSERT(still_idle < pages / 10,
"vCPU%d: Too many pages still idle (%"PRIu64 " out of %"
PRIu64 ").\n",
vcpu_id, still_idle, pages);
static void assert_ucall(struct kvm_vm *vm, uint32_t vcpu_id,
uint64_t expected_ucall)
struct ucall uc;
uint64_t actual_ucall = get_ucall(vm, vcpu_id, &uc);
TEST_ASSERT(expected_ucall == actual_ucall,
"Guest exited unexpectedly (expected ucall %" PRIu64
", got %" PRIu64 ")",
expected_ucall, actual_ucall);
static bool spin_wait_for_next_iteration(int *current_iteration)
int last_iteration = *current_iteration;
do {
if (READ_ONCE(done))
return false;
*current_iteration = READ_ONCE(iteration);
} while (last_iteration == *current_iteration);
return true;
static void vcpu_thread_main(struct perf_test_vcpu_args *vcpu_args)
struct kvm_vm *vm = perf_test_args.vm;
int vcpu_id = vcpu_args->vcpu_id;
int current_iteration = 0;
while (spin_wait_for_next_iteration(&current_iteration)) {
switch (READ_ONCE(iteration_work)) {
vcpu_run(vm, vcpu_id);
assert_ucall(vm, vcpu_id, UCALL_SYNC);
mark_vcpu_memory_idle(vm, vcpu_id);
vcpu_last_completed_iteration[vcpu_id] = current_iteration;
static void spin_wait_for_vcpu(int vcpu_id, int target_iteration)
while (READ_ONCE(vcpu_last_completed_iteration[vcpu_id]) !=
target_iteration) {
/* The type of memory accesses to perform in the VM. */
enum access_type {
static void run_iteration(struct kvm_vm *vm, int vcpus, const char *description)
struct timespec ts_start;
struct timespec ts_elapsed;
int next_iteration;
int vcpu_id;
/* Kick off the vCPUs by incrementing iteration. */
next_iteration = ++iteration;
clock_gettime(CLOCK_MONOTONIC, &ts_start);
/* Wait for all vCPUs to finish the iteration. */
for (vcpu_id = 0; vcpu_id < vcpus; vcpu_id++)
spin_wait_for_vcpu(vcpu_id, next_iteration);
ts_elapsed = timespec_elapsed(ts_start);
pr_info("%-30s: %ld.%09lds\n",
description, ts_elapsed.tv_sec, ts_elapsed.tv_nsec);
static void access_memory(struct kvm_vm *vm, int vcpus, enum access_type access,
const char *description)
perf_test_set_wr_fract(vm, (access == ACCESS_READ) ? INT_MAX : 1);
run_iteration(vm, vcpus, description);
static void mark_memory_idle(struct kvm_vm *vm, int vcpus)
* Even though this parallelizes the work across vCPUs, this is still a
* very slow operation because page_idle forces the test to mark one pfn
* at a time and the clear_young notifier serializes on the KVM MMU
* lock.
pr_debug("Marking VM memory idle (slow)...\n");
iteration_work = ITERATION_MARK_IDLE;
run_iteration(vm, vcpus, "Mark memory idle");
static void run_test(enum vm_guest_mode mode, void *arg)
struct test_params *params = arg;
struct kvm_vm *vm;
int vcpus = params->vcpus;
vm = perf_test_create_vm(mode, vcpus, params->vcpu_memory_bytes, 1,
params->backing_src, !overlap_memory_access);
perf_test_start_vcpu_threads(vcpus, vcpu_thread_main);
access_memory(vm, vcpus, ACCESS_WRITE, "Populating memory");
/* As a control, read and write to the populated memory first. */
access_memory(vm, vcpus, ACCESS_WRITE, "Writing to populated memory");
access_memory(vm, vcpus, ACCESS_READ, "Reading from populated memory");
/* Repeat on memory that has been marked as idle. */
mark_memory_idle(vm, vcpus);
access_memory(vm, vcpus, ACCESS_WRITE, "Writing to idle memory");
mark_memory_idle(vm, vcpus);
access_memory(vm, vcpus, ACCESS_READ, "Reading from idle memory");
/* Set done to signal the vCPU threads to exit */
done = true;
static void help(char *name)
printf("usage: %s [-h] [-m mode] [-b vcpu_bytes] [-v vcpus] [-o] [-s mem_type]\n",
printf(" -h: Display this help message.");
printf(" -b: specify the size of the memory region which should be\n"
" dirtied by each vCPU. e.g. 10M or 3G.\n"
" (default: 1G)\n");
printf(" -v: specify the number of vCPUs to run.\n");
printf(" -o: Overlap guest memory accesses instead of partitioning\n"
" them into a separate region of memory for each vCPU.\n");
int main(int argc, char *argv[])
struct test_params params = {
.backing_src = DEFAULT_VM_MEM_SRC,
.vcpu_memory_bytes = DEFAULT_PER_VCPU_MEM_SIZE,
.vcpus = 1,
int page_idle_fd;
int opt;
while ((opt = getopt(argc, argv, "hm:b:v:os:")) != -1) {
switch (opt) {
case 'm':
case 'b':
params.vcpu_memory_bytes = parse_size(optarg);
case 'v':
params.vcpus = atoi(optarg);
case 'o':
overlap_memory_access = true;
case 's':
params.backing_src = parse_backing_src_type(optarg);
case 'h':
page_idle_fd = open("/sys/kernel/mm/page_idle/bitmap", O_RDWR);
if (page_idle_fd < 0) {
print_skip("CONFIG_IDLE_PAGE_TRACKING is not enabled");
for_each_guest_mode(run_test, &params);
return 0;