// 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 (for example
* in nesting, where TLB size is unlimited) this test will print a warning
* 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 "memstress.h"
#include "guest_modes.h"
#include "processor.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. */
ITERATION_ACCESS_MEMORY,
/* Mark the vCPU's memory idle in page_idle. */
ITERATION_MARK_IDLE,
} iteration_work;
/* 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 nr_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;
__TEST_REQUIRE(pfn, "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,
struct memstress_vcpu_args *vcpu_args)
{
int vcpu_idx = vcpu_args->vcpu_idx;
uint64_t base_gva = vcpu_args->gva;
uint64_t pages = vcpu_args->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_idx)
return;
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 * memstress_args.guest_page_size;
uint64_t pfn = lookup_pfn(pagemap_fd, vm, gva);
if (!pfn) {
no_pfn++;
continue;
}
if (is_page_idle(page_idle_fd, pfn)) {
still_idle++;
continue;
}
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_idx, no_pfn, pages);
/*
* Check 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.
*
* 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) {
#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);
}
static void assert_ucall(struct kvm_vcpu *vcpu, uint64_t expected_ucall)
{
struct ucall uc;
uint64_t actual_ucall = get_ucall(vcpu, &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(memstress_args.stop_vcpus))
return false;
*current_iteration = READ_ONCE(iteration);
} while (last_iteration == *current_iteration);
return true;
}
static void vcpu_thread_main(struct memstress_vcpu_args *vcpu_args)
{
struct kvm_vcpu *vcpu = vcpu_args->vcpu;
struct kvm_vm *vm = memstress_args.vm;
int vcpu_idx = vcpu_args->vcpu_idx;
int current_iteration = 0;
while (spin_wait_for_next_iteration(¤t_iteration)) {
switch (READ_ONCE(iteration_work)) {
case ITERATION_ACCESS_MEMORY:
vcpu_run(vcpu);
assert_ucall(vcpu, UCALL_SYNC);
break;
case ITERATION_MARK_IDLE:
mark_vcpu_memory_idle(vm, vcpu_args);
break;
};
vcpu_last_completed_iteration[vcpu_idx] = current_iteration;
}
}
static void spin_wait_for_vcpu(int vcpu_idx, int target_iteration)
{
while (READ_ONCE(vcpu_last_completed_iteration[vcpu_idx]) !=
target_iteration) {
continue;
}
}
/* The type of memory accesses to perform in the VM. */
enum access_type {
ACCESS_READ,
ACCESS_WRITE,
};
static void run_iteration(struct kvm_vm *vm, int nr_vcpus, const char *description)
{
struct timespec ts_start;
struct timespec ts_elapsed;
int next_iteration, i;
/* 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 (i = 0; i < nr_vcpus; i++)
spin_wait_for_vcpu(i, 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 nr_vcpus,
enum access_type access, const char *description)
{
memstress_set_write_percent(vm, (access == ACCESS_READ) ? 0 : 100);
iteration_work = ITERATION_ACCESS_MEMORY;
run_iteration(vm, nr_vcpus, description);
}
static void mark_memory_idle(struct kvm_vm *vm, int nr_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, nr_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 nr_vcpus = params->nr_vcpus;
vm = memstress_create_vm(mode, nr_vcpus, params->vcpu_memory_bytes, 1,
params->backing_src, !overlap_memory_access);
memstress_start_vcpu_threads(nr_vcpus, vcpu_thread_main);
pr_info("\n");
access_memory(vm, nr_vcpus, ACCESS_WRITE, "Populating memory");
/* As a control, read and write to the populated memory first. */
access_memory(vm, nr_vcpus, ACCESS_WRITE, "Writing to populated memory");
access_memory(vm, nr_vcpus, ACCESS_READ, "Reading from populated memory");
/* Repeat on memory that has been marked as idle. */
mark_memory_idle(vm, nr_vcpus);
access_memory(vm, nr_vcpus, ACCESS_WRITE, "Writing to idle memory");
mark_memory_idle(vm, nr_vcpus);
access_memory(vm, nr_vcpus, ACCESS_READ, "Reading from idle memory");
memstress_join_vcpu_threads(nr_vcpus);
memstress_destroy_vm(vm);
}
static void help(char *name)
{
puts("");
printf("usage: %s [-h] [-m mode] [-b vcpu_bytes] [-v vcpus] [-o] [-s mem_type]\n",
name);
puts("");
printf(" -h: Display this help message.");
guest_modes_help();
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");
backing_src_help("-s");
puts("");
exit(0);
}
int main(int argc, char *argv[])
{
struct test_params params = {
.backing_src = DEFAULT_VM_MEM_SRC,
.vcpu_memory_bytes = DEFAULT_PER_VCPU_MEM_SIZE,
.nr_vcpus = 1,
};
int page_idle_fd;
int opt;
guest_modes_append_default();
while ((opt = getopt(argc, argv, "hm:b:v:os:")) != -1) {
switch (opt) {
case 'm':
guest_modes_cmdline(optarg);
break;
case 'b':
params.vcpu_memory_bytes = parse_size(optarg);
break;
case 'v':
params.nr_vcpus = atoi_positive("Number of vCPUs", optarg);
break;
case 'o':
overlap_memory_access = true;
break;
case 's':
params.backing_src = parse_backing_src_type(optarg);
break;
case 'h':
default:
help(argv[0]);
break;
}
}
page_idle_fd = open("/sys/kernel/mm/page_idle/bitmap", O_RDWR);
__TEST_REQUIRE(page_idle_fd >= 0,
"CONFIG_IDLE_PAGE_TRACKING is not enabled");
close(page_idle_fd);
for_each_guest_mode(run_test, ¶ms);
return 0;
}
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