On Fri, Oct 03, 2014 at 06:47:50PM +0100, Dr. David Alan Gilbert (git) wrote: > From: "Dr. David Alan Gilbert" <dgilb...@redhat.com> > > userfaultfd is a Linux syscall that gives an fd that receives a stream > of notifications of accesses to pages marked as MADV_USERFAULT, and > allows the program to acknowledge those stalls and tell the accessing > thread to carry on. > > Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
[snip]
> /*
> + * Tell the kernel that we've now got some memory it previously asked for.
> + * Note: We're not allowed to ack a page which wasn't requested.
> + */
> +static int ack_userfault(MigrationIncomingState *mis, void *start, size_t
> len)
> +{
> + uint64_t tmp[2];
> +
> + /*
> + * Kernel wants the range that's now safe to access
> + * Note it always takes 64bit values, even on a 32bit host.
> + */
> + tmp[0] = (uint64_t)(uintptr_t)start;
> + tmp[1] = (uint64_t)(uintptr_t)start + (uint64_t)len;
> +
> + if (write(mis->userfault_fd, tmp, 16) != 16) {
> + int e = errno;
Is an EOF (i.e. write() returns 0) ever possible here? If so errno
may not have a meaningful value.
> + if (e == ENOENT) {
> + /* Kernel said it wasn't waiting - one case where this can
> + * happen is where two threads triggered the userfault
> + * and we receive the page and ack it just after we received
> + * the 2nd request and that ends up deciding it should ack it
> + * We could optimise it out, but it's rare.
> + */
> + /*fprintf(stderr, "ack_userfault: %p/%zx ENOENT\n", start, len);
> */
> + return 0;
> + }
> + error_report("postcopy_ram: Failed to notify kernel for %p/%zx (%d)",
> + start, len, e);
> + return -errno;
> + }
> +
> + return 0;
> +}
> +
> +/*
> * Handle faults detected by the USERFAULT markings
> */
> static void *postcopy_ram_fault_thread(void *opaque)
> {
> MigrationIncomingState *mis = (MigrationIncomingState *)opaque;
> + void *hostaddr;
> + int ret;
> + size_t hostpagesize = getpagesize();
> + RAMBlock *rb = NULL;
> + RAMBlock *last_rb = NULL; /* last RAMBlock we sent part of */
>
> - fprintf(stderr, "postcopy_ram_fault_thread\n");
> - /* TODO: In later patch */
> + DPRINTF("%s", __func__);
> qemu_sem_post(&mis->fault_thread_sem);
> - while (1) {
> - /* TODO: In later patch */
> - }
> + while (true) {
> + PostcopyPMIState old_state, tmp_state;
> + ram_addr_t rb_offset;
> + ram_addr_t in_raspace;
> + unsigned long bitmap_index;
> + struct pollfd pfd[2];
> +
> + /*
> + * We're mainly waiting for the kernel to give us a faulting HVA,
> + * however we can be told to quit via userfault_quit_fd which is
> + * an eventfd
> + */
> + pfd[0].fd = mis->userfault_fd;
> + pfd[0].events = POLLIN;
> + pfd[0].revents = 0;
> + pfd[1].fd = mis->userfault_quit_fd;
> + pfd[1].events = POLLIN; /* Waiting for eventfd to go positive */
> + pfd[1].revents = 0;
> +
> + if (poll(pfd, 2, -1 /* Wait forever */) == -1) {
> + perror("userfault poll");
> + break;
> + }
>
> + if (pfd[1].revents) {
> + DPRINTF("%s got quit event", __func__);
> + break;
I don't see any cleanup path in the userfault thread. So wouldn't it
be simpler to just pthread_cancel() it rather than using an extra fd
for quit notifications.
> + }
> +
> + ret = read(mis->userfault_fd, &hostaddr, sizeof(hostaddr));
> + if (ret != sizeof(hostaddr)) {
> + if (ret < 0) {
> + perror("Failed to read full userfault hostaddr");
> + break;
> + } else {
> + error_report("%s: Read %d bytes from userfaultfd expected
> %zd",
> + __func__, ret, sizeof(hostaddr));
> + break; /* Lost alignment, don't know what we'd read next */
> + }
> + }
> +
> + rb = qemu_ram_block_from_host(hostaddr, true, &in_raspace,
> &rb_offset,
> + &bitmap_index);
> + if (!rb) {
> + error_report("postcopy_ram_fault_thread: Fault outside guest:
> %p",
> + hostaddr);
> + break;
> + }
> +
> + DPRINTF("%s: Request for HVA=%p index=%lx rb=%s offset=%zx",
> + __func__, hostaddr, bitmap_index, qemu_ram_get_idstr(rb),
> + rb_offset);
> +
> + tmp_state = postcopy_pmi_get_state(mis, bitmap_index);
> + do {
> + old_state = tmp_state;
> +
> + switch (old_state) {
> + case POSTCOPY_PMI_REQUESTED:
> + /* Do nothing - it's already requested */
> + break;
> +
> + case POSTCOPY_PMI_RECEIVED:
> + /* Already arrived - no state change, just kick the kernel */
> + DPRINTF("postcopy_ram_fault_thread: notify pre of %p",
> + hostaddr);
> + if (ack_userfault(mis,
> + (void *)((uintptr_t)hostaddr
> + & ~(hostpagesize - 1)),
> + hostpagesize)) {
> + assert(0);
> + }
> + break;
> +
> + case POSTCOPY_PMI_MISSING:
> +
> + tmp_state = postcopy_pmi_change_state(mis, bitmap_index,
> + old_state,
> POSTCOPY_PMI_REQUESTED);
> + if (tmp_state == POSTCOPY_PMI_MISSING) {
> + /*
> + * Send the request to the source - we want to request
> one
> + * of our host page sizes (which is >= TPS)
> + */
> + if (rb != last_rb) {
> + last_rb = rb;
> + migrate_send_rp_reqpages(mis, qemu_ram_get_idstr(rb),
> + rb_offset, hostpagesize);
> + } else {
> + /* Save some space */
> + migrate_send_rp_reqpages(mis, NULL,
> + rb_offset, hostpagesize);
> + }
> + }
> + break;
> + }
> + } while (tmp_state != old_state);
> + }
> + DPRINTF("%s: exit", __func__);
> return NULL;
> }
>
> int postcopy_ram_enable_notify(MigrationIncomingState *mis)
> {
> - /* Create the fault handler thread and wait for it to be ready */
> + uint64_t tmp64;
> +
> + /* Open the fd for the kernel to give us userfaults */
> + mis->userfault_fd = syscall(__NR_userfaultfd, O_CLOEXEC);
> + if (mis->userfault_fd == -1) {
> + perror("Failed to open userfault fd");
> + return -1;
> + }
> +
> + /*
> + * Version handshake, we send it the version we want and expect to get
> the
> + * same back.
> + */
> + tmp64 = USERFAULTFD_PROTOCOL;
> + if (write(mis->userfault_fd, &tmp64, sizeof(tmp64)) != sizeof(tmp64)) {
> + perror("Writing userfaultfd version");
> + close(mis->userfault_fd);
> + return -1;
> + }
> + if (read(mis->userfault_fd, &tmp64, sizeof(tmp64)) != sizeof(tmp64)) {
> + perror("Reading userfaultfd version");
> + close(mis->userfault_fd);
> + return -1;
> + }
> + if (tmp64 != USERFAULTFD_PROTOCOL) {
> + error_report("Mismatched userfaultfd version, expected %zx, got %zx",
> + (size_t)USERFAULTFD_PROTOCOL, (size_t)tmp64);
> + close(mis->userfault_fd);
> + return -1;
> + }
> +
> + /* Now an eventfd we use to tell the fault-thread to quit */
> + mis->userfault_quit_fd = eventfd(0, EFD_CLOEXEC);
> + if (mis->userfault_quit_fd == -1) {
> + perror("Opening userfault_quit_fd");
> + close(mis->userfault_fd);
> + return -1;
> + }
> +
> qemu_sem_init(&mis->fault_thread_sem, 0);
> qemu_thread_create(&mis->fault_thread, "postcopy/fault",
> postcopy_ram_fault_thread, mis, QEMU_THREAD_JOINABLE);
> qemu_sem_wait(&mis->fault_thread_sem);
> + mis->have_fault_thread = true;
>
> /* Mark so that we get notified of accesses to unwritten areas */
> if (qemu_ram_foreach_block(postcopy_ram_sensitise_area, mis)) {
> return -1;
> }
>
> + DPRINTF("postcopy_ram_enable_notify: Sensitised");
> +
> return 0;
> }
>
> @@ -612,11 +814,12 @@ int postcopy_place_page(MigrationIncomingState *mis,
> void *host, void *from,
>
> if (syscall(__NR_remap_anon_pages, host, from, hps, 0) !=
> getpagesize()) {
> + int e = errno;
> perror("remap_anon_pages in postcopy_place_page");
> fprintf(stderr, "host: %p from: %p pmi=%d\n", host, from,
> postcopy_pmi_get_state(mis, bitmap_offset));
>
> - return -errno;
> + return -e;
Unrelated change, should probably be folded into the patch which added
this code.
> }
>
> tmp_state = postcopy_pmi_get_state(mis, bitmap_offset);
> @@ -629,7 +832,10 @@ int postcopy_place_page(MigrationIncomingState *mis,
> void *host, void *from,
>
>
> if (old_state == POSTCOPY_PMI_REQUESTED) {
> - /* TODO: Notify kernel */
> + /* Send the kernel the host address that should now be accessible */
> + DPRINTF("%s: Notifying kernel bitmap_offset=0x%lx host=%p",
> + __func__, bitmap_offset, host);
> + return ack_userfault(mis, host, hps);
> }
>
> return 0;
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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