Re: [Qemu-devel] [PATCH v6 42/47] Postcopy; Handle userfault requests

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* zhanghailiang (zhang.zhanghailiang@huawei.com) wrote:
> On 2015/4/15 1:04, Dr. David Alan Gilbert (git) wrote:
> >From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >
> >userfaultfd is a Linux syscall that gives an fd that receives a stream
> >of notifications of accesses to pages registered with it and allows
> >the program to acknowledge those stalls and tell the accessing
> >thread to carry on.
> >
> >Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >---
> >  include/migration/migration.h |   4 +
> >  migration/postcopy-ram.c      | 165 +++++++++++++++++++++++++++++++++++++++---
> >  trace-events                  |   9 +++
> >  3 files changed, 169 insertions(+), 9 deletions(-)
> >
> >diff --git a/include/migration/migration.h b/include/migration/migration.h
> >index db06fd2..4d6f33a 100644
> >--- a/include/migration/migration.h
> >+++ b/include/migration/migration.h
> >@@ -75,11 +75,15 @@ struct MigrationIncomingState {
> >       */
> >      QemuEvent      main_thread_load_event;
> >
> >+    bool           have_fault_thread;
> >      QemuThread     fault_thread;
> >      QemuSemaphore  fault_thread_sem;
> >
> >      /* For the kernel to send us notifications */
> >      int            userfault_fd;
> >+    /* To tell the fault_thread to quit */
> >+    int            userfault_quit_fd;
> >+
> >      QEMUFile *return_path;
> >      QemuMutex      rp_mutex;    /* We send replies from multiple threads */
> >      void          *postcopy_tmp_page;
> >diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> >index 33aadbc..b2dc3b7 100644
> >--- a/migration/postcopy-ram.c
> >+++ b/migration/postcopy-ram.c
> >@@ -49,6 +49,8 @@ struct PostcopyDiscardState {
> >   */
> >  #if defined(__linux__)
> >
> >+#include <poll.h>
> >+#include <sys/eventfd.h>
> >  #include <sys/mman.h>
> >  #include <sys/ioctl.h>
> >  #include <sys/syscall.h>
> >@@ -273,15 +275,41 @@ int postcopy_ram_incoming_init(MigrationIncomingState *mis, size_t ram_pages)
> >   */
> >  int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
> >  {
> >-    /* TODO: Join the fault thread once we're sure it will exit */
> >-    if (qemu_ram_foreach_block(cleanup_area, mis)) {
> >-        return -1;
> >+    trace_postcopy_ram_incoming_cleanup_entry();
> >+
> >+    if (mis->have_fault_thread) {
> >+        uint64_t tmp64;
> >+
> >+        if (qemu_ram_foreach_block(cleanup_area, mis)) {
> >+            return -1;
> >+        }
> >+        /*
> >+         * Tell the fault_thread to exit, it's an eventfd that should
> >+         * currently be at 0, we're going to inc it to 1
> >+         */
> >+        tmp64 = 1;
> >+        if (write(mis->userfault_quit_fd, &tmp64, 8) == 8) {
> >+            trace_postcopy_ram_incoming_cleanup_join();
> >+            qemu_thread_join(&mis->fault_thread);
> >+        } else {
> >+            /* Not much we can do here, but may as well report it */
> >+            error_report("%s: incing userfault_quit_fd: %s", __func__,
> >+                         strerror(errno));
> >+        }
> >+        trace_postcopy_ram_incoming_cleanup_closeuf();
> >+        close(mis->userfault_fd);
> >+        close(mis->userfault_quit_fd);
> >+        mis->have_fault_thread = false;
> >      }
> >
> >+    postcopy_state_set(mis, POSTCOPY_INCOMING_END);
> >+    migrate_send_rp_shut(mis, qemu_file_get_error(mis->file) != 0);
> >+
> >      if (mis->postcopy_tmp_page) {
> >          munmap(mis->postcopy_tmp_page, getpagesize());
> >          mis->postcopy_tmp_page = NULL;
> >      }
> >+    trace_postcopy_ram_incoming_cleanup_exit();
> >      return 0;
> >  }
> >
> >@@ -320,31 +348,150 @@ static int ram_block_enable_notify(const char *block_name, void *host_addr,
> >  static void *postcopy_ram_fault_thread(void *opaque)
> >  {
> >      MigrationIncomingState *mis = (MigrationIncomingState *)opaque;
> >-
> >-    fprintf(stderr, "postcopy_ram_fault_thread\n");
> >-    /* TODO: In later patch */
> >+    uint64_t hostaddr; /* The kernel always gives us 64 bit, not a pointer */
> >+    int ret;
> >+    size_t hostpagesize = getpagesize();
> >+    RAMBlock *rb = NULL;
> >+    RAMBlock *last_rb = NULL; /* last RAMBlock we sent part of */
> >+    uint8_t *local_tmp_page;
> >+
> >+    trace_postcopy_ram_fault_thread_entry();
> >      qemu_sem_post(&mis->fault_thread_sem);
> >-    while (1) {
> >-        /* TODO: In later patch */
> >+
> >+    local_tmp_page = mmap(NULL, getpagesize(),
> >+                          PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS,
> >+                          -1, 0);
> >+    if (!local_tmp_page) {
> >+        error_report("%s mapping local tmp page: %s", __func__,
> >+                     strerror(errno));
> >+        return NULL;
> >      }
> >+    if (madvise(local_tmp_page, getpagesize(), MADV_DONTFORK)) {
> 
> What's this 'local_tmp_page' used for ? I don't find where it is used in this function.

Yes, that was old code; it's already gone.

> Besides, there is a helper function qemu_madvise() in qemu, maybe you should use it :)

Thanks;  I could do for this one but I've got to be careful;
qemu_madvise is mainly for portability, and this code is Linux specific
anyway, and some of the QEMU_MADV_ flags aren't valid for all of the OSs
anyway.

Dave

> 
> Thanks.
> 
> >+        munmap(local_tmp_page, getpagesize());
> >+        error_report("%s postcopy local page DONTFORK: %s", __func__,
> >+                     strerror(errno));
> >+        return NULL;
> >+    }
> >+
> >+    while (true) {
> >+        ram_addr_t rb_offset;
> >+        ram_addr_t in_raspace;
> >+        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) {
> >+            error_report("%s: userfault poll: %s", __func__, strerror(errno));
> >+            break;
> >+        }
> >+
> >+        if (pfd[1].revents) {
> >+            trace_postcopy_ram_fault_thread_quit();
> >+            break;
> >+        }
> >+
> >+        ret = read(mis->userfault_fd, &hostaddr, sizeof(hostaddr));
> >+        if (ret != sizeof(hostaddr)) {
> >+            if (errno == EAGAIN) {
> >+                /*
> >+                 * if a wake up happens on the other thread just after
> >+                 * the poll, there is nothing to read.
> >+                 */
> >+                continue;
> >+            }
> >+            if (ret < 0) {
> >+                error_report("%s: Failed to read full userfault hostaddr: %s",
> >+                             __func__, strerror(errno));
> >+                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((void *)(uintptr_t)hostaddr, true,
> >+                                      &in_raspace, &rb_offset);
> >+        if (!rb) {
> >+            error_report("postcopy_ram_fault_thread: Fault outside guest: %"
> >+                         PRIx64, hostaddr);
> >+            break;
> >+        }
> >+
> >+        trace_postcopy_ram_fault_thread_request(hostaddr,
> >+                                                qemu_ram_get_idstr(rb),
> >+                                                rb_offset);
> >+
> >+        /*
> >+         * 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_req_pages(mis, qemu_ram_get_idstr(rb),
> >+                                     rb_offset, hostpagesize);
> >+        } else {
> >+            /* Save some space */
> >+            migrate_send_rp_req_pages(mis, NULL,
> >+                                     rb_offset, hostpagesize);
> >+        }
> >+    }
> >+    munmap(local_tmp_page, getpagesize());
> >+    trace_postcopy_ram_fault_thread_exit();
> >      return NULL;
> >  }
> >
> >  int postcopy_ram_enable_notify(MigrationIncomingState *mis)
> >  {
> >-    /* Create the fault handler thread and wait for it to be ready */
> >+    /* Open the fd for the kernel to give us userfaults */
> >+    mis->userfault_fd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
> >+    if (mis->userfault_fd == -1) {
> >+        error_report("%s: Failed to open userfault fd: %s", __func__,
> >+                     strerror(errno));
> >+        return -1;
> >+    }
> >+
> >+    /*
> >+     * Although the host check already tested the API, we need to
> >+     * do the check again as an ABI handshake on the new fd.
> >+     */
> >+    if (!ufd_version_check(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) {
> >+        error_report("%s: Opening userfault_quit_fd: %s", __func__,
> >+                     strerror(errno));
> >+        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);
> >      qemu_sem_destroy(&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(ram_block_enable_notify, mis)) {
> >          return -1;
> >      }
> >
> >+    trace_postcopy_ram_enable_notify();
> >+
> >      return 0;
> >  }
> >
> >diff --git a/trace-events b/trace-events
> >index b65740c..72a65fa 100644
> >--- a/trace-events
> >+++ b/trace-events
> >@@ -1500,6 +1500,15 @@ postcopy_cleanup_area(const char *ramblock, void *host_addr, size_t offset, size
> >  postcopy_ram_discard_range(void *start, void *end) "%p,%p"
> >  postcopy_init_area(const char *ramblock, void *host_addr, size_t offset, size_t length) "%s: %p offset=%zx length=%zx"
> >  postcopy_place_page(void *host_addr, bool all_zero) "host=%p all_zero=%d"
> >+postcopy_ram_enable_notify(void) ""
> >+postcopy_ram_fault_thread_entry(void) ""
> >+postcopy_ram_fault_thread_exit(void) ""
> >+postcopy_ram_fault_thread_quit(void) ""
> >+postcopy_ram_fault_thread_request(uint64_t hostaddr, const char *ramblock, size_t offset) "Request for HVA=%" PRIx64 " rb=%s offset=%zx"
> >+postcopy_ram_incoming_cleanup_closeuf(void) ""
> >+postcopy_ram_incoming_cleanup_entry(void) ""
> >+postcopy_ram_incoming_cleanup_exit(void) ""
> >+postcopy_ram_incoming_cleanup_join(void) ""
> >
> >  # kvm-all.c
> >  kvm_ioctl(int type, void *arg) "type 0x%x, arg %p"
> >
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK