KVM: selftests: Add test to populate a VM with the max possible guest mem

Add a selftest that enables populating a VM with the maximum amount of
guest memory allowed by the underlying architecture.  Abuse KVM's
memslots by mapping a single host memory region into multiple memslots so
that the selftest doesn't require a system with terabytes of RAM.

Default to 512gb of guest memory, which isn't all that interesting, but
should work on all MMUs and doesn't take an exorbitant amount of memory
or time.  E.g. testing with ~64tb of guest memory takes the better part
of an hour, and requires 200gb of memory for KVM's page tables when using
4kb pages.

To inflicit maximum abuse on KVM' MMU, default to 4kb pages (or whatever
the not-hugepage size is) in the backing store (memfd).  Use memfd for
the host backing store to ensure that hugepages are guaranteed when
requested, and to give the user explicit control of the size of hugepage
being tested.

By default, spin up as many vCPUs as there are available to the selftest,
and distribute the work of dirtying each 4kb chunk of memory across all
vCPUs.  Dirtying guest memory forces KVM to populate its page tables, and
also forces KVM to write back accessed/dirty information to struct page
when the guest memory is freed.

On x86, perform two passes with a MMU context reset between each pass to
coerce KVM into dropping all references to the MMU root, e.g. to emulate
a vCPU dropping the last reference.  Perform both passes and all
rendezvous on all architectures in the hope that arm64 and s390x can gain
similar shenanigans in the future.

Measure and report the duration of each operation, which is helpful not
only to verify the test is working as intended, but also to easily
evaluate the performance differences different page sizes.

Provide command line options to limit the amount of guest memory, set the
size of each slot (i.e. of the host memory region), set the number of
vCPUs, and to enable usage of hugepages.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220226001546.360188-29-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/.gitignore

@@ -58,6 +58,7 @@
 /hardware_disable_test
 /kvm_create_max_vcpus
 /kvm_page_table_test
+/max_guest_memory_test
 /memslot_modification_stress_test
 /memslot_perf_test
 /rseq_test

tools/testing/selftests/kvm/Makefile

@@ -93,6 +93,7 @@ TEST_GEN_PROGS_x86_64 += dirty_log_perf_test
 TEST_GEN_PROGS_x86_64 += hardware_disable_test
 TEST_GEN_PROGS_x86_64 += kvm_create_max_vcpus
 TEST_GEN_PROGS_x86_64 += kvm_page_table_test
+TEST_GEN_PROGS_x86_64 += max_guest_memory_test
 TEST_GEN_PROGS_x86_64 += memslot_modification_stress_test
 TEST_GEN_PROGS_x86_64 += memslot_perf_test
 TEST_GEN_PROGS_x86_64 += rseq_test

tools/testing/selftests/kvm/max_guest_memory_test.c

+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <semaphore.h>
+#include <sys/types.h>
+#include <signal.h>
+#include <errno.h>
+#include <linux/bitmap.h>
+#include <linux/bitops.h>
+#include <linux/atomic.h>
+
+#include "kvm_util.h"
+#include "test_util.h"
+#include "guest_modes.h"
+#include "processor.h"
+
+static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
+{
+	uint64_t gpa;
+
+	for (gpa = start_gpa; gpa < end_gpa; gpa += stride)
+		*((volatile uint64_t *)gpa) = gpa;
+
+	GUEST_DONE();
+}
+
+struct vcpu_info {
+	struct kvm_vm *vm;
+	uint32_t id;
+	uint64_t start_gpa;
+	uint64_t end_gpa;
+};
+
+static int nr_vcpus;
+static atomic_t rendezvous;
+
+static void rendezvous_with_boss(void)
+{
+	int orig = atomic_read(&rendezvous);
+
+	if (orig > 0) {
+		atomic_dec_and_test(&rendezvous);
+		while (atomic_read(&rendezvous) > 0)
+			cpu_relax();
+	} else {
+		atomic_inc(&rendezvous);
+		while (atomic_read(&rendezvous) < 0)
+			cpu_relax();
+	}
+}
+
+static void run_vcpu(struct kvm_vm *vm, uint32_t vcpu_id)
+{
+	vcpu_run(vm, vcpu_id);
+	ASSERT_EQ(get_ucall(vm, vcpu_id, NULL), UCALL_DONE);
+}
+
+static void *vcpu_worker(void *data)
+{
+	struct vcpu_info *vcpu = data;
+	struct kvm_vm *vm = vcpu->vm;
+	struct kvm_sregs sregs;
+	struct kvm_regs regs;
+
+	vcpu_args_set(vm, vcpu->id, 3, vcpu->start_gpa, vcpu->end_gpa,
+		      vm_get_page_size(vm));
+
+	/* Snapshot regs before the first run. */
+	vcpu_regs_get(vm, vcpu->id, &regs);
+	rendezvous_with_boss();
+
+	run_vcpu(vm, vcpu->id);
+	rendezvous_with_boss();
+	vcpu_regs_set(vm, vcpu->id, &regs);
+	vcpu_sregs_get(vm, vcpu->id, &sregs);
+#ifdef __x86_64__
+	/* Toggle CR0.WP to trigger a MMU context reset. */
+	sregs.cr0 ^= X86_CR0_WP;
+#endif
+	vcpu_sregs_set(vm, vcpu->id, &sregs);
+	rendezvous_with_boss();
+
+	run_vcpu(vm, vcpu->id);
+	rendezvous_with_boss();
+
+	return NULL;
+}
+
+static pthread_t *spawn_workers(struct kvm_vm *vm, uint64_t start_gpa,
+				uint64_t end_gpa)
+{
+	struct vcpu_info *info;
+	uint64_t gpa, nr_bytes;
+	pthread_t *threads;
+	int i;
+
+	threads = malloc(nr_vcpus * sizeof(*threads));
+	TEST_ASSERT(threads, "Failed to allocate vCPU threads");
+
+	info = malloc(nr_vcpus * sizeof(*info));
+	TEST_ASSERT(info, "Failed to allocate vCPU gpa ranges");
+
+	nr_bytes = ((end_gpa - start_gpa) / nr_vcpus) &
+			~((uint64_t)vm_get_page_size(vm) - 1);
+	TEST_ASSERT(nr_bytes, "C'mon, no way you have %d CPUs", nr_vcpus);
+
+	for (i = 0, gpa = start_gpa; i < nr_vcpus; i++, gpa += nr_bytes) {
+		info[i].vm = vm;
+		info[i].id = i;
+		info[i].start_gpa = gpa;
+		info[i].end_gpa = gpa + nr_bytes;
+		pthread_create(&threads[i], NULL, vcpu_worker, &info[i]);
+	}
+	return threads;
+}
+
+static void rendezvous_with_vcpus(struct timespec *time, const char *name)
+{
+	int i, rendezvoused;
+
+	pr_info("Waiting for vCPUs to finish %s...\n", name);
+
+	rendezvoused = atomic_read(&rendezvous);
+	for (i = 0; abs(rendezvoused) != 1; i++) {
+		usleep(100);
+		if (!(i & 0x3f))
+			pr_info("\r%d vCPUs haven't rendezvoused...",
+				abs(rendezvoused) - 1);
+		rendezvoused = atomic_read(&rendezvous);
+	}
+
+	clock_gettime(CLOCK_MONOTONIC, time);
+
+	/* Release the vCPUs after getting the time of the previous action. */
+	pr_info("\rAll vCPUs finished %s, releasing...\n", name);
+	if (rendezvoused > 0)
+		atomic_set(&rendezvous, -nr_vcpus - 1);
+	else
+		atomic_set(&rendezvous, nr_vcpus + 1);
+}
+
+static void calc_default_nr_vcpus(void)
+{
+	cpu_set_t possible_mask;
+	int r;
+
+	r = sched_getaffinity(0, sizeof(possible_mask), &possible_mask);
+	TEST_ASSERT(!r, "sched_getaffinity failed, errno = %d (%s)",
+		    errno, strerror(errno));
+
+	nr_vcpus = CPU_COUNT(&possible_mask) * 3/4;
+	TEST_ASSERT(nr_vcpus > 0, "Uh, no CPUs?");
+}
+
+int main(int argc, char *argv[])
+{
+	/*
+	 * Skip the first 4gb and slot0.  slot0 maps <1gb and is used to back
+	 * the guest's code, stack, and page tables.  Because selftests creates
+	 * an IRQCHIP, a.k.a. a local APIC, KVM creates an internal memslot
+	 * just below the 4gb boundary.  This test could create memory at
+	 * 1gb-3gb,but it's simpler to skip straight to 4gb.
+	 */
+	const uint64_t size_1gb = (1 << 30);
+	const uint64_t start_gpa = (4ull * size_1gb);
+	const int first_slot = 1;
+
+	struct timespec time_start, time_run1, time_reset, time_run2;
+	uint64_t max_gpa, gpa, slot_size, max_mem, i;
+	int max_slots, slot, opt, fd;
+	bool hugepages = false;
+	pthread_t *threads;
+	struct kvm_vm *vm;
+	void *mem;
+
+	/*
+	 * Default to 2gb so that maxing out systems with MAXPHADDR=46, which
+	 * are quite common for x86, requires changing only max_mem (KVM allows
+	 * 32k memslots, 32k * 2gb == ~64tb of guest memory).
+	 */
+	slot_size = 2 * size_1gb;
+
+	max_slots = kvm_check_cap(KVM_CAP_NR_MEMSLOTS);
+	TEST_ASSERT(max_slots > first_slot, "KVM is broken");
+
+	/* All KVM MMUs should be able to survive a 128gb guest. */
+	max_mem = 128 * size_1gb;
+
+	calc_default_nr_vcpus();
+
+	while ((opt = getopt(argc, argv, "c:h:m:s:H")) != -1) {
+		switch (opt) {
+		case 'c':
+			nr_vcpus = atoi(optarg);
+			TEST_ASSERT(nr_vcpus > 0, "number of vcpus must be >0");
+			break;
+		case 'm':
+			max_mem = atoi(optarg) * size_1gb;
+			TEST_ASSERT(max_mem > 0, "memory size must be >0");
+			break;
+		case 's':
+			slot_size = atoi(optarg) * size_1gb;
+			TEST_ASSERT(slot_size > 0, "slot size must be >0");
+			break;
+		case 'H':
+			hugepages = true;
+			break;
+		case 'h':
+		default:
+			printf("usage: %s [-c nr_vcpus] [-m max_mem_in_gb] [-s slot_size_in_gb] [-H]\n", argv[0]);
+			exit(1);
+		}
+	}
+
+	vm = vm_create_default_with_vcpus(nr_vcpus, 0, 0, guest_code, NULL);
+
+	max_gpa = vm_get_max_gfn(vm) << vm_get_page_shift(vm);
+	TEST_ASSERT(max_gpa > (4 * slot_size), "MAXPHYADDR <4gb ");
+
+	fd = kvm_memfd_alloc(slot_size, hugepages);
+	mem = mmap(NULL, slot_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	TEST_ASSERT(mem != MAP_FAILED, "mmap() failed");
+
+	TEST_ASSERT(!madvise(mem, slot_size, MADV_NOHUGEPAGE), "madvise() failed");
+
+	/* Pre-fault the memory to avoid taking mmap_sem on guest page faults. */
+	for (i = 0; i < slot_size; i += vm_get_page_size(vm))
+		((uint8_t *)mem)[i] = 0xaa;
+
+	gpa = 0;
+	for (slot = first_slot; slot < max_slots; slot++) {
+		gpa = start_gpa + ((slot - first_slot) * slot_size);
+		if (gpa + slot_size > max_gpa)
+			break;
+
+		if ((gpa - start_gpa) >= max_mem)
+			break;
+
+		vm_set_user_memory_region(vm, slot, 0, gpa, slot_size, mem);
+
+#ifdef __x86_64__
+		/* Identity map memory in the guest using 1gb pages. */
+		for (i = 0; i < slot_size; i += size_1gb)
+			__virt_pg_map(vm, gpa + i, gpa + i, X86_PAGE_SIZE_1G);
+#else
+		for (i = 0; i < slot_size; i += vm_get_page_size(vm))
+			virt_pg_map(vm, gpa + i, gpa + i);
+#endif
+	}
+
+	atomic_set(&rendezvous, nr_vcpus + 1);
+	threads = spawn_workers(vm, start_gpa, gpa);
+
+	pr_info("Running with %lugb of guest memory and %u vCPUs\n",
+		(gpa - start_gpa) / size_1gb, nr_vcpus);
+
+	rendezvous_with_vcpus(&time_start, "spawning");
+	rendezvous_with_vcpus(&time_run1, "run 1");
+	rendezvous_with_vcpus(&time_reset, "reset");
+	rendezvous_with_vcpus(&time_run2, "run 2");
+
+	time_run2  = timespec_sub(time_run2,   time_reset);
+	time_reset = timespec_sub(time_reset, time_run1);
+	time_run1  = timespec_sub(time_run1,   time_start);
+
+	pr_info("run1 = %ld.%.9lds, reset = %ld.%.9lds, run2 =  %ld.%.9lds\n",
+		time_run1.tv_sec, time_run1.tv_nsec,
+		time_reset.tv_sec, time_reset.tv_nsec,
+		time_run2.tv_sec, time_run2.tv_nsec);
+
+	/*
+	 * Delete even numbered slots (arbitrary) and unmap the first half of
+	 * the backing (also arbitrary) to verify KVM correctly drops all
+	 * references to the removed regions.
+	 */
+	for (slot = (slot - 1) & ~1ull; slot >= first_slot; slot -= 2)
+		vm_set_user_memory_region(vm, slot, 0, 0, 0, NULL);
+
+	munmap(mem, slot_size / 2);
+
+	/* Sanity check that the vCPUs actually ran. */
+	for (i = 0; i < nr_vcpus; i++)
+		pthread_join(threads[i], NULL);
+
+	/*
+	 * Deliberately exit without deleting the remaining memslots or closing
+	 * kvm_fd to test cleanup via mmu_notifier.release.
+	 */
+}