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KVM: selftests: allow using UFFD minor faults for demand paging
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UFFD handling of MINOR faults is a new feature whose use case is to
speed up demand paging (compared to MISSING faults). So, it's
interesting to let this selftest exercise this new mode.

Modify the demand paging test to have the option of using UFFD minor
faults, as opposed to missing faults. Now, when turning on userfaultfd
with '-u', the desired mode has to be specified ("MISSING" or "MINOR").

If we're in minor mode, before registering, prefault via the *alias*.
This way, the guest will trigger minor faults, instead of missing
faults, and we can UFFDIO_CONTINUE to resolve them.

Modify the page fault handler function to use the right ioctl depending
on the mode we're running in. In MINOR mode, use UFFDIO_CONTINUE.

Signed-off-by: Axel Rasmussen <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
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@CmdrMoozy @bonzini
CmdrMoozy authored and bonzini committed May 27, 2021
1 parent 94f3f2b commit a4b9722
Showing 1 changed file with 79 additions and 33 deletions.
112 changes: 79 additions & 33 deletions tools/testing/selftests/kvm/demand_paging_test.c
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Original file line number Diff line number Diff line change
Expand Up @@ -73,33 +73,48 @@ static void *vcpu_worker(void *data)
return NULL;
}

static int handle_uffd_page_request(int uffd, uint64_t addr)
static int handle_uffd_page_request(int uffd_mode, int uffd, uint64_t addr)
{
pid_t tid;
pid_t tid = syscall(__NR_gettid);
struct timespec start;
struct timespec ts_diff;
struct uffdio_copy copy;
int r;

tid = syscall(__NR_gettid);
clock_gettime(CLOCK_MONOTONIC, &start);

copy.src = (uint64_t)guest_data_prototype;
copy.dst = addr;
copy.len = demand_paging_size;
copy.mode = 0;
if (uffd_mode == UFFDIO_REGISTER_MODE_MISSING) {
struct uffdio_copy copy;

clock_gettime(CLOCK_MONOTONIC, &start);
copy.src = (uint64_t)guest_data_prototype;
copy.dst = addr;
copy.len = demand_paging_size;
copy.mode = 0;

r = ioctl(uffd, UFFDIO_COPY, &copy);
if (r == -1) {
pr_info("Failed UFFDIO_COPY in 0x%lx from thread %d with errno: %d\n",
addr, tid, errno);
return r;
}
} else if (uffd_mode == UFFDIO_REGISTER_MODE_MINOR) {
struct uffdio_continue cont = {0};

cont.range.start = addr;
cont.range.len = demand_paging_size;

r = ioctl(uffd, UFFDIO_COPY, &copy);
if (r == -1) {
pr_info("Failed Paged in 0x%lx from thread %d with errno: %d\n",
addr, tid, errno);
return r;
r = ioctl(uffd, UFFDIO_CONTINUE, &cont);
if (r == -1) {
pr_info("Failed UFFDIO_CONTINUE in 0x%lx from thread %d with errno: %d\n",
addr, tid, errno);
return r;
}
} else {
TEST_FAIL("Invalid uffd mode %d", uffd_mode);
}

ts_diff = timespec_elapsed(start);

PER_PAGE_DEBUG("UFFDIO_COPY %d \t%ld ns\n", tid,
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);
Expand All @@ -110,6 +125,7 @@ static int handle_uffd_page_request(int uffd, uint64_t addr)
bool quit_uffd_thread;

struct uffd_handler_args {
int uffd_mode;
int uffd;
int pipefd;
useconds_t delay;
Expand Down Expand Up @@ -186,7 +202,7 @@ static void *uffd_handler_thread_fn(void *arg)
if (delay)
usleep(delay);
addr = msg.arg.pagefault.address;
r = handle_uffd_page_request(uffd, addr);
r = handle_uffd_page_request(uffd_args->uffd_mode, uffd, addr);
if (r < 0)
return NULL;
pages++;
Expand All @@ -202,13 +218,32 @@ static void *uffd_handler_thread_fn(void *arg)

static void setup_demand_paging(struct kvm_vm *vm,
pthread_t *uffd_handler_thread, int pipefd,
useconds_t uffd_delay,
int uffd_mode, useconds_t uffd_delay,
struct uffd_handler_args *uffd_args,
void *hva, uint64_t len)
void *hva, void *alias, uint64_t len)
{
bool is_minor = (uffd_mode == UFFDIO_REGISTER_MODE_MINOR);
int uffd;
struct uffdio_api uffdio_api;
struct uffdio_register uffdio_register;
uint64_t expected_ioctls = ((uint64_t) 1) << _UFFDIO_COPY;

PER_PAGE_DEBUG("Userfaultfd %s mode, faults resolved with %s\n",
is_minor ? "MINOR" : "MISSING",
is_minor ? "UFFDIO_CONINUE" : "UFFDIO_COPY");

/* In order to get minor faults, prefault via the alias. */
if (is_minor) {
size_t p;

expected_ioctls = ((uint64_t) 1) << _UFFDIO_CONTINUE;

TEST_ASSERT(alias != NULL, "Alias required for minor faults");
for (p = 0; p < (len / demand_paging_size); ++p) {
memcpy(alias + (p * demand_paging_size),
guest_data_prototype, demand_paging_size);
}
}

uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
TEST_ASSERT(uffd >= 0, "uffd creation failed, errno: %d", errno);
Expand All @@ -221,12 +256,13 @@ static void setup_demand_paging(struct kvm_vm *vm,

uffdio_register.range.start = (uint64_t)hva;
uffdio_register.range.len = len;
uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
uffdio_register.mode = uffd_mode;
TEST_ASSERT(ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) != -1,
"ioctl UFFDIO_REGISTER failed");
TEST_ASSERT((uffdio_register.ioctls & UFFD_API_RANGE_IOCTLS) ==
UFFD_API_RANGE_IOCTLS, "unexpected userfaultfd ioctl set");
TEST_ASSERT((uffdio_register.ioctls & expected_ioctls) ==
expected_ioctls, "missing userfaultfd ioctls");

uffd_args->uffd_mode = uffd_mode;
uffd_args->uffd = uffd;
uffd_args->pipefd = pipefd;
uffd_args->delay = uffd_delay;
Expand All @@ -238,7 +274,7 @@ static void setup_demand_paging(struct kvm_vm *vm,
}

struct test_params {
bool use_uffd;
int uffd_mode;
useconds_t uffd_delay;
enum vm_mem_backing_src_type src_type;
bool partition_vcpu_memory_access;
Expand Down Expand Up @@ -275,7 +311,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
perf_test_setup_vcpus(vm, nr_vcpus, guest_percpu_mem_size,
p->partition_vcpu_memory_access);

if (p->use_uffd) {
if (p->uffd_mode) {
uffd_handler_threads =
malloc(nr_vcpus * sizeof(*uffd_handler_threads));
TEST_ASSERT(uffd_handler_threads, "Memory allocation failed");
Expand All @@ -289,6 +325,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
for (vcpu_id = 0; vcpu_id < nr_vcpus; vcpu_id++) {
vm_paddr_t vcpu_gpa;
void *vcpu_hva;
void *vcpu_alias;
uint64_t vcpu_mem_size;


Expand All @@ -303,8 +340,9 @@ static void run_test(enum vm_guest_mode mode, void *arg)
PER_VCPU_DEBUG("Added VCPU %d with test mem gpa [%lx, %lx)\n",
vcpu_id, vcpu_gpa, vcpu_gpa + vcpu_mem_size);

/* Cache the HVA pointer of the region */
/* Cache the host addresses of the region */
vcpu_hva = addr_gpa2hva(vm, vcpu_gpa);
vcpu_alias = addr_gpa2alias(vm, vcpu_gpa);

/*
* Set up user fault fd to handle demand paging
Expand All @@ -315,8 +353,9 @@ static void run_test(enum vm_guest_mode mode, void *arg)
TEST_ASSERT(!r, "Failed to set up pipefd");

setup_demand_paging(vm, &uffd_handler_threads[vcpu_id],
pipefds[vcpu_id * 2], p->uffd_delay,
&uffd_args[vcpu_id], vcpu_hva,
pipefds[vcpu_id * 2], p->uffd_mode,
p->uffd_delay, &uffd_args[vcpu_id],
vcpu_hva, vcpu_alias,
vcpu_mem_size);
}
}
Expand Down Expand Up @@ -345,7 +384,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)

pr_info("All vCPU threads joined\n");

if (p->use_uffd) {
if (p->uffd_mode) {
char c;

/* Tell the user fault fd handler threads to quit */
Expand All @@ -367,7 +406,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)

free(guest_data_prototype);
free(vcpu_threads);
if (p->use_uffd) {
if (p->uffd_mode) {
free(uffd_handler_threads);
free(uffd_args);
free(pipefds);
Expand All @@ -377,11 +416,11 @@ static void run_test(enum vm_guest_mode mode, void *arg)
static void help(char *name)
{
puts("");
printf("usage: %s [-h] [-m mode] [-u] [-d uffd_delay_usec]\n"
printf("usage: %s [-h] [-m vm_mode] [-u uffd_mode] [-d uffd_delay_usec]\n"
" [-b memory] [-t type] [-v vcpus] [-o]\n", name);
guest_modes_help();
printf(" -u: use User Fault FD to handle vCPU page\n"
" faults.\n");
printf(" -u: use userfaultfd to handle vCPU page faults. Mode is a\n"
" UFFD registration mode: 'MISSING' or 'MINOR'.\n");
printf(" -d: add a delay in usec to the User Fault\n"
" FD handler to simulate demand paging\n"
" overheads. Ignored without -u.\n");
Expand All @@ -408,13 +447,17 @@ int main(int argc, char *argv[])

guest_modes_append_default();

while ((opt = getopt(argc, argv, "hm:ud:b:t:v:o")) != -1) {
while ((opt = getopt(argc, argv, "hm:u:d:b:t:v:o")) != -1) {
switch (opt) {
case 'm':
guest_modes_cmdline(optarg);
break;
case 'u':
p.use_uffd = true;
if (!strcmp("MISSING", optarg))
p.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'.");
break;
case 'd':
p.uffd_delay = strtoul(optarg, NULL, 0);
Expand All @@ -441,6 +484,9 @@ int main(int argc, char *argv[])
}
}

TEST_ASSERT(p.uffd_mode != UFFDIO_REGISTER_MODE_MINOR || p.src_type == VM_MEM_SRC_SHMEM,
"userfaultfd MINOR mode requires shared memory; pick a different -t");

for_each_guest_mode(run_test, &p);

return 0;
Expand Down

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