Demonstrate a (very basic) scheme for supporting memory fault exits.
>From the vCPU threads:
1. Simply issue UFFDIO_COPY/CONTINUEs in response to memory fault exits,
with the purpose of establishing the absent mappings. Do so with
wake_waiters=false to avoid serializing on the userfaultfd wait queue
locks.
2. When the UFFDIO_COPY/CONTINUE in (1) fails with EEXIST,
assume that the mapping was already established but is currently
absent [A] and attempt to populate it using MADV_POPULATE_WRITE.
Issue UFFDIO_COPY/CONTINUEs from the reader threads as well, but with
wake_waiters=true to ensure that any threads sleeping on the uffd are
eventually woken up.
A real VMM would track whether it had already COPY/CONTINUEd pages (eg,
via a bitmap) to avoid calls destined to EEXIST. However, even the
naive approach is enough to demonstrate the performance advantages of
KVM_EXIT_MEMORY_FAULT.
[A] In reality it is much likelier that the vCPU thread simply lost a
race to establish the mapping for the page.
Signed-off-by: Anish Moorthy <amoorthy@xxxxxxxxxx>
Acked-by: James Houghton <jthoughton@xxxxxxxxxx>
---
.../selftests/kvm/demand_paging_test.c | 220 +++++++++++++-----
1 file changed, 164 insertions(+), 56 deletions(-)
diff --git a/tools/testing/selftests/kvm/demand_paging_test.c b/tools/testing/selftests/kvm/demand_paging_test.c
index 607cd2846e39c..dce72adcb1632 100644
--- a/tools/testing/selftests/kvm/demand_paging_test.c
+++ b/tools/testing/selftests/kvm/demand_paging_test.c
@@ -15,6 +15,7 @@
#include <time.h>
#include <pthread.h>
#include <linux/userfaultfd.h>
+#include <sys/mman.h>
#include <sys/syscall.h>
#include "kvm_util.h"
@@ -31,6 +32,60 @@ static uint64_t guest_percpu_mem_size = DEFAULT_PER_VCPU_MEM_SIZE;
static size_t demand_paging_size;
static char *guest_data_prototype;
+static int num_uffds;
+static size_t uffd_region_size;
+static struct uffd_desc **uffd_descs;
+/*
+ * Delay when demand paging is performed through userfaultfd or directly by
+ * vcpu_worker in the case of a KVM_EXIT_MEMORY_FAULT.
+ */
+static useconds_t uffd_delay;
+static int uffd_mode;
+
+
+static int handle_uffd_page_request(
+ int uffd_mode, int uffd, uint64_t hva, bool is_vcpu
+);
+
+static void madv_write_or_err(uint64_t gpa)
+{
+ int r;
+ void *hva = addr_gpa2hva(memstress_args.vm, gpa);
+
+ r = madvise(hva, demand_paging_size, MADV_POPULATE_WRITE);
+ TEST_ASSERT(
+ r == 0,
+ "MADV_POPULATE_WRITE on hva 0x%lx (gpa 0x%lx) failed with errno %i\n",
+ (uintptr_t) hva, gpa, errno);
+}
+
+static void ready_page(uint64_t gpa)
+{
+ int r, uffd;
+
+ /*
+ * This test only registers memslot 1 w/ userfaultfd. Any accesses outside
+ * the registered ranges should fault in the physical pages through
+ * MADV_POPULATE_WRITE.
+ */
+ if ((gpa < memstress_args.gpa)
+ || (gpa >= memstress_args.gpa + memstress_args.size)) {
+ madv_write_or_err(gpa);
+ } else {
+ if (uffd_delay)
+ usleep(uffd_delay);
+
+ uffd = uffd_descs[(gpa - memstress_args.gpa) / uffd_region_size]->uffd;
+
+ r = handle_uffd_page_request(
+ uffd_mode, uffd,
+ (uint64_t) addr_gpa2hva(memstress_args.vm, gpa), true);
+
+ if (r == EEXIST)
+ madv_write_or_err(gpa);
+ }
+}
+
static void vcpu_worker(struct memstress_vcpu_args *vcpu_args)
{
struct kvm_vcpu *vcpu = vcpu_args->vcpu;
@@ -42,25 +97,37 @@ static void vcpu_worker(struct memstress_vcpu_args *vcpu_args)
clock_gettime(CLOCK_MONOTONIC, &start);
- /* Let the guest access its memory */
- ret = _vcpu_run(vcpu);
- TEST_ASSERT(ret == 0, "vcpu_run failed: %d\n", ret);
- if (get_ucall(vcpu, NULL) != UCALL_SYNC) {
- TEST_ASSERT(false,
- "Invalid guest sync status: exit_reason=%s\n",
- exit_reason_str(run->exit_reason));
- }
+ while (true) {
+ /* Let the guest access its memory */
+ ret = _vcpu_run(vcpu);
+ TEST_ASSERT(ret == 0 || (run->exit_reason == KVM_EXIT_MEMORY_FAULT),
+ "vcpu_run failed: %d\n", ret);
+ if (get_ucall(vcpu, NULL) != UCALL_SYNC) {
+
+ if (run->exit_reason == KVM_EXIT_MEMORY_FAULT) {
+ TEST_ASSERT(run->memory_fault.flags == 0,
+ "Unrecognized flags 0x%llx on memory fault exit",
+ run->memory_fault.flags);
+ ready_page(run->memory_fault.gpa);
+ continue;
+ }
+
+ TEST_ASSERT(false,
+ "Invalid guest sync status: exit_reason=%s\n",
+ exit_reason_str(run->exit_reason));
+ }
- ts_diff = timespec_elapsed(start);
- PER_VCPU_DEBUG("vCPU %d execution time: %ld.%.9lds\n", vcpu_idx,
- ts_diff.tv_sec, ts_diff.tv_nsec);
+ ts_diff = timespec_elapsed(start);
+ PER_VCPU_DEBUG("vCPU %d execution time: %ld.%.9lds\n", vcpu_idx,
+ ts_diff.tv_sec, ts_diff.tv_nsec);
+ break;
+ }
}
-static int handle_uffd_page_request(int uffd_mode, int uffd,
- struct uffd_msg *msg)
+static int handle_uffd_page_request(
+ int uffd_mode, int uffd, uint64_t hva, bool is_vcpu)
{
pid_t tid = syscall(__NR_gettid);
- uint64_t addr = msg->arg.pagefault.address;
struct timespec start;
struct timespec ts_diff;
int r;
@@ -71,58 +138,81 @@ static int handle_uffd_page_request(int uffd_mode, int uffd,
struct uffdio_copy copy;
copy.src = (uint64_t)guest_data_prototype;
- copy.dst = addr;
+ copy.dst = hva;
copy.len = demand_paging_size;
- copy.mode = 0;
+ copy.mode = UFFDIO_COPY_MODE_DONTWAKE;
- r = ioctl(uffd, UFFDIO_COPY, ©);
/*
- * With multiple vCPU threads fault on a single page and there are
- * multiple readers for the UFFD, at least one of the UFFDIO_COPYs
- * will fail with EEXIST: handle that case without signaling an
- * error.
+ * With multiple vCPU threads and at least one of multiple reader threads
+ * or vCPU memory faults, multiple vCPUs accessing an absent page will
+ * almost certainly cause some thread doing the UFFDIO_COPY here to get
+ * EEXIST: make sure to allow that case.
*/
- if (r == -1 && errno != EEXIST) {
- pr_info(
- "Failed UFFDIO_COPY in 0x%lx from thread %d, errno = %d\n",
- addr, tid, errno);
- return r;
- }
+ r = ioctl(uffd, UFFDIO_COPY, ©);
+ TEST_ASSERT(
+ r == 0 || errno == EEXIST,
+ "Thread 0x%x failed UFFDIO_COPY on hva 0x%lx, errno = %d",
+ gettid(), hva, errno);
} else if (uffd_mode == UFFDIO_REGISTER_MODE_MINOR) {
+ /* The comments in the UFFDIO_COPY branch also apply here. */
struct uffdio_continue cont = {0};
- cont.range.start = addr;
+ cont.range.start = hva;
cont.range.len = demand_paging_size;
+ cont.mode = UFFDIO_CONTINUE_MODE_DONTWAKE;
r = ioctl(uffd, UFFDIO_CONTINUE, &cont);
- /* See the note about EEXISTs in the UFFDIO_COPY branch. */
- if (r == -1 && errno != EEXIST) {
- pr_info(
- "Failed UFFDIO_CONTINUE in 0x%lx from thread %d, errno = %d\n",
- addr, tid, errno);
- return r;
- }
+ TEST_ASSERT(
+ r == 0 || errno == EEXIST,
+ "Thread 0x%x failed UFFDIO_CONTINUE on hva 0x%lx, errno = %d",
+ gettid(), hva, errno);
} else {
TEST_FAIL("Invalid uffd mode %d", uffd_mode);
}
+ /*
+ * If the above UFFDIO_COPY/CONTINUE fails with EEXIST, it will do so without
+ * waking threads waiting on the UFFD: make sure that happens here.
+ */
+ if (!is_vcpu) {
+ struct uffdio_range range = {
+ .start = hva,
+ .len = demand_paging_size
+ };
+ r = ioctl(uffd, UFFDIO_WAKE, &range);
+ TEST_ASSERT(
+ r == 0,
+ "Thread 0x%x failed UFFDIO_WAKE on hva 0x%lx, errno = %d",
+ gettid(), hva, errno);
+ }
+
ts_diff = timespec_elapsed(start);
PER_PAGE_DEBUG("UFFD page-in %d \t%ld ns\n", tid,
timespec_to_ns(ts_diff));
PER_PAGE_DEBUG("Paged in %ld bytes at 0x%lx from thread %d\n",
- demand_paging_size, addr, tid);
+ demand_paging_size, hva, tid);
return 0;
}
+static int handle_uffd_page_request_from_uffd(
+ int uffd_mode, int uffd, struct uffd_msg *msg)
+{
+ TEST_ASSERT(
+ msg->event == UFFD_EVENT_PAGEFAULT,
+ "Received uffd message with event %d != UFFD_EVENT_PAGEFAULT",
+ msg->event);
+ return handle_uffd_page_request(
+ uffd_mode, uffd, msg->arg.pagefault.address, false);
+}
+
struct test_params {
- int uffd_mode;
bool single_uffd;
- useconds_t uffd_delay;
int readers_per_uffd;
enum vm_mem_backing_src_type src_type;
bool partition_vcpu_memory_access;
+ bool memfault_exits;
};
static void prefault_mem(void *alias, uint64_t len)
@@ -139,18 +229,31 @@ static void prefault_mem(void *alias, uint64_t len)
static void run_test(enum vm_guest_mode mode, void *arg)
{
struct test_params *p = arg;
- struct uffd_desc **uffd_descs = NULL;
struct timespec start;
struct timespec ts_diff;
struct kvm_vm *vm;
- int i, num_uffds = 0;
- uint64_t uffd_region_size;
+ int i;
+ uint32_t slot_flags = 0;
+ bool uffd_memfault_exits = uffd_mode && p->memfault_exits;
+
+ if (uffd_memfault_exits) {
+ TEST_ASSERT(kvm_has_cap(KVM_CAP_MEMORY_FAULT_NOWAIT) > 0,
+ "KVM does not have KVM_CAP_MEMORY_FAULT_NOWAIT");
+ slot_flags = KVM_MEM_ABSENT_MAPPING_FAULT;
+ }
vm = memstress_create_vm(
mode, nr_vcpus, guest_percpu_mem_size,
- 1, 0,
+ 1, slot_flags,
p->src_type, p->partition_vcpu_memory_access);
+ if (uffd_memfault_exits) {
+ if (kvm_has_cap(KVM_CAP_X86_MEMORY_FAULT_EXIT))
+ vm_enable_cap(
+ vm, KVM_CAP_X86_MEMORY_FAULT_EXIT,
+ KVM_MEMFAULT_REASON_ABSENT_MAPPING);
+ }
+
demand_paging_size = get_backing_src_pagesz(p->src_type);
guest_data_prototype = malloc(demand_paging_size);
@@ -158,12 +261,12 @@ static void run_test(enum vm_guest_mode mode, void *arg)
"Failed to allocate buffer for guest data pattern");
memset(guest_data_prototype, 0xAB, demand_paging_size);
- if (p->uffd_mode) {
+ if (uffd_mode) {
num_uffds = p->single_uffd ? 1 : nr_vcpus;
uffd_region_size = nr_vcpus * guest_percpu_mem_size / num_uffds;
uffd_descs = malloc(num_uffds * sizeof(struct uffd_desc *));
- TEST_ASSERT(uffd_descs, "Memory allocation failed");
+ TEST_ASSERT(uffd_descs, "Failed to allocate memory of uffd descriptors");
for (i = 0; i < num_uffds; i++) {
struct memstress_vcpu_args *vcpu_args;
@@ -183,10 +286,10 @@ static void run_test(enum vm_guest_mode mode, void *arg)
* requests.
*/
uffd_descs[i] = uffd_setup_demand_paging(
- p->uffd_mode, p->uffd_delay, vcpu_hva,
+ uffd_mode, uffd_delay, vcpu_hva,
uffd_region_size,
p->readers_per_uffd,
- &handle_uffd_page_request);
+ &handle_uffd_page_request_from_uffd);
}
}
@@ -200,7 +303,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
ts_diff = timespec_elapsed(start);
pr_info("All vCPU threads joined\n");
- if (p->uffd_mode) {
+ if (uffd_mode) {
/* Tell the user fault fd handler threads to quit */
for (i = 0; i < num_uffds; i++)
uffd_stop_demand_paging(uffd_descs[i]);
@@ -215,7 +318,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
memstress_destroy_vm(vm);
free(guest_data_prototype);
- if (p->uffd_mode)
+ if (uffd_mode)
free(uffd_descs);
}
@@ -224,7 +327,7 @@ static void help(char *name)
puts("");
printf("usage: %s [-h] [-m vm_mode] [-u uffd_mode] [-a]\n"
" [-d uffd_delay_usec] [-r readers_per_uffd] [-b memory]\n"
- " [-s type] [-v vcpus] [-o]\n", name);
+ " [-w] [-s type] [-v vcpus] [-o]\n", name);
guest_modes_help();
printf(" -u: use userfaultfd to handle vCPU page faults. Mode is a\n"
" UFFD registration mode: 'MISSING' or 'MINOR'.\n");
@@ -235,6 +338,7 @@ static void help(char *name)
" FD handler to simulate demand paging\n"
" overheads. Ignored without -u.\n");
printf(" -r: Set the number of reader threads per uffd.\n");
+ printf(" -w: Enable kvm cap for memory fault exits.\n");
printf(" -b: specify the size of the memory region which should be\n"
" demand paged by each vCPU. e.g. 10M or 3G.\n"
" Default: 1G\n");
@@ -254,29 +358,30 @@ int main(int argc, char *argv[])
.partition_vcpu_memory_access = true,
.readers_per_uffd = 1,
.single_uffd = false,
+ .memfault_exits = false,
};
int opt;
guest_modes_append_default();
- while ((opt = getopt(argc, argv, "ahom:u:d:b:s:v:r:")) != -1) {
+ while ((opt = getopt(argc, argv, "ahowm:u:d:b:s:v:r:")) != -1) {
switch (opt) {
case 'm':
guest_modes_cmdline(optarg);
break;
case 'u':
if (!strcmp("MISSING", optarg))
- p.uffd_mode = UFFDIO_REGISTER_MODE_MISSING;
+ uffd_mode = UFFDIO_REGISTER_MODE_MISSING;
else if (!strcmp("MINOR", optarg))
- p.uffd_mode = UFFDIO_REGISTER_MODE_MINOR;
- TEST_ASSERT(p.uffd_mode, "UFFD mode must be 'MISSING' or 'MINOR'.");
+ uffd_mode = UFFDIO_REGISTER_MODE_MINOR;
+ TEST_ASSERT(uffd_mode, "UFFD mode must be 'MISSING' or 'MINOR'.");
break;
case 'a':
p.single_uffd = true;
break;
case 'd':
- p.uffd_delay = strtoul(optarg, NULL, 0);
- TEST_ASSERT(p.uffd_delay >= 0, "A negative UFFD delay is not supported.");
+ uffd_delay = strtoul(optarg, NULL, 0);
+ TEST_ASSERT(uffd_delay >= 0, "A negative UFFD delay is not supported.");
break;
case 'b':
guest_percpu_mem_size = parse_size(optarg);
@@ -299,6 +404,9 @@ int main(int argc, char *argv[])
"Invalid number of readers per uffd %d: must be >=1",
p.readers_per_uffd);
break;
+ case 'w':
+ p.memfault_exits = true;
+ break;
case 'h':
default:
help(argv[0]);
@@ -306,7 +414,7 @@ int main(int argc, char *argv[])
}
}
- if (p.uffd_mode == UFFDIO_REGISTER_MODE_MINOR &&
+ if (uffd_mode == UFFDIO_REGISTER_MODE_MINOR &&
!backing_src_is_shared(p.src_type)) {
TEST_FAIL("userfaultfd MINOR mode requires shared memory; pick a different -s");
}
--
2.40.0.rc1.284.g88254d51c5-goog
