Re: [PATCH v4 2/6] introduce UFFD-WP low-level interface helpers

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Re: [PATCH v4 2/6] introduce UFFD-WP low-level interface helpers

From:	Andrey Gruzdev
Subject:	Re: [PATCH v4 2/6] introduce UFFD-WP low-level interface helpers
Date:	Tue, 1 Dec 2020 22:32:12 +0300
User-agent:	Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0

On 01.12.2020 15:24, Dr. David Alan Gilbert wrote:

* Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:

Implemented support for the whole RAM block memory
protection/un-protection. Introduced higher level
ram_write_tracking_start() and ram_write_tracking_stop()
to start/stop tracking guest memory writes.

Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
---
  include/exec/memory.h      |   7 ++
  include/qemu/userfaultfd.h |  29 +++++
  migration/ram.c            | 120 +++++++++++++++++++++
  migration/ram.h            |   4 +
  util/meson.build           |   1 +
  util/userfaultfd.c         | 215 +++++++++++++++++++++++++++++++++++++
  6 files changed, 376 insertions(+)
  create mode 100644 include/qemu/userfaultfd.h
  create mode 100644 util/userfaultfd.c

diff --git a/include/exec/memory.h b/include/exec/memory.h
index 0f3e6bcd5e..3d798fce16 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -139,6 +139,13 @@ typedef struct IOMMUNotifier IOMMUNotifier;
  /* RAM is a persistent kind memory */
  #define RAM_PMEM (1 << 5)

+/*

+ * UFFDIO_WRITEPROTECT is used on this RAMBlock to
+ * support 'write-tracking' migration type.
+ * Implies ram_state->ram_wt_enabled.
+ */
+#define RAM_UF_WRITEPROTECT (1 << 6)
+
  static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn,
                                         IOMMUNotifierFlag flags,
                                         hwaddr start, hwaddr end,
diff --git a/include/qemu/userfaultfd.h b/include/qemu/userfaultfd.h
new file mode 100644
index 0000000000..fb843c76db
--- /dev/null
+++ b/include/qemu/userfaultfd.h
@@ -0,0 +1,29 @@
+/*
+ * Linux UFFD-WP support
+ *
+ * Copyright Virtuozzo GmbH, 2020
+ *
+ * Authors:
+ *  Andrey Gruzdev   <andrey.gruzdev@virtuozzo.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or
+ * later.  See the COPYING file in the top-level directory.
+ */
+
+#ifndef USERFAULTFD_H
+#define USERFAULTFD_H
+
+#include "qemu/osdep.h"
+#include "exec/hwaddr.h"
+#include <linux/userfaultfd.h>
+
+int uffd_create_fd(void);
+void uffd_close_fd(int uffd);
+int uffd_register_memory(int uffd, hwaddr start, hwaddr length,
+        bool track_missing, bool track_wp);
+int uffd_unregister_memory(int uffd, hwaddr start, hwaddr length);
+int uffd_protect_memory(int uffd, hwaddr start, hwaddr length, bool wp);
+int uffd_read_events(int uffd, struct uffd_msg *msgs, int count);
+bool uffd_poll_events(int uffd, int tmo);
+
+#endif /* USERFAULTFD_H */
diff --git a/migration/ram.c b/migration/ram.c
index 7811cde643..3adfd1948d 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -56,6 +56,11 @@
  #include "savevm.h"
  #include "qemu/iov.h"
  #include "multifd.h"
+#include "sysemu/runstate.h"
+
+#ifdef CONFIG_LINUX
+#include "qemu/userfaultfd.h"
+#endif

/***********************************************************/

  /* ram save/restore */
@@ -298,6 +303,8 @@ struct RAMSrcPageRequest {
  struct RAMState {
      /* QEMUFile used for this migration */
      QEMUFile *f;
+    /* UFFD file descriptor, used in 'write-tracking' migration */
+    int uffdio_fd;
      /* Last block that we have visited searching for dirty pages */
      RAMBlock *last_seen_block;
      /* Last block from where we have sent data */
@@ -3788,6 +3795,119 @@ static int ram_resume_prepare(MigrationState *s, void 
*opaque)
      return 0;
  }

+/*

+ * ram_write_tracking_start: start UFFD-WP memory tracking
+ *
+ * Returns 0 for success or negative value in case of error
+ *
+ */
+int ram_write_tracking_start(void)


Please split this stuff off into a separate patch; just make this one
the userfaultfd.[ch] and then a separate one for gluing it into ram.c

Yep, I'm separating it now - userfaultdf.c and ram_write_tracking stuff,they go to different patches.

+{
+#ifdef CONFIG_LINUX
+    int uffd;
+    RAMState *rs = ram_state;
+    RAMBlock *bs;
+
+    /* Open UFFD file descriptor */
+    uffd = uffd_create_fd();
+    if (uffd < 0) {
+        return uffd;
+    }
+    rs->uffdio_fd = uffd;
+
+    RAMBLOCK_FOREACH_NOT_IGNORED(bs) {
+        /* Nothing to do with read-only and MMIO-writable regions */
+        if (bs->mr->readonly || bs->mr->rom_device) {
+            continue;
+        }
+
+        bs->flags |= RAM_UF_WRITEPROTECT;
+        /* Register block memory with UFFD to track writes */
+        if (uffd_register_memory(rs->uffdio_fd, (hwaddr) bs->host,
+                bs->max_length, false, true)) {
+            goto fail;
+        }
+        /* Apply UFFD write protection to the block memory range */
+        if (uffd_protect_memory(rs->uffdio_fd, (hwaddr) bs->host,
+                bs->max_length, true)) {
+            goto fail;
+        }
+
+        info_report("UFFD-WP write-tracking enabled: "
+                "block_id=%s page_size=%zu start=%p length=%lu "
+                "romd_mode=%i ram=%i readonly=%i nonvolatile=%i rom_device=%i",
+                bs->idstr, bs->page_size, bs->host, bs->max_length,
+                bs->mr->romd_mode, bs->mr->ram, bs->mr->readonly,
+                bs->mr->nonvolatile, bs->mr->rom_device);
+    }
+
+    return 0;
+
+fail:
+    error_report("ram_write_tracking_start() failed: restoring initial memory 
state");
+
+    RAMBLOCK_FOREACH_NOT_IGNORED(bs) {
+        if ((bs->flags & RAM_UF_WRITEPROTECT) == 0) {
+            continue;
+        }
+        /*
+         * In case some memory block failed to be write-protected
+         * remove protection and unregister all succeeded RAM blocks
+         */
+        uffd_protect_memory(rs->uffdio_fd, (hwaddr) bs->host, bs->max_length, 
false);
+        uffd_unregister_memory(rs->uffdio_fd, (hwaddr) bs->host, 
bs->max_length);
+        /* Cleanup flags */
+        bs->flags &= ~RAM_UF_WRITEPROTECT;
+    }
+
+    uffd_close_fd(uffd);
+    rs->uffdio_fd = -1;
+    return -1;
+#else
+    rs->uffdio_fd = -1;
+    error_setg(&migrate_get_current()->error,
+            "Background-snapshot not supported on non-Linux hosts");
+    return -1;
+#endif /* CONFIG_LINUX */
+}
+
+/**
+ * ram_write_tracking_stop: stop UFFD-WP memory tracking and remove protection
+ */
+void ram_write_tracking_stop(void)
+{
+#ifdef CONFIG_LINUX
+    RAMState *rs = ram_state;
+    RAMBlock *bs;
+    assert(rs->uffdio_fd >= 0);
+
+    RAMBLOCK_FOREACH_NOT_IGNORED(bs) {
+        if ((bs->flags & RAM_UF_WRITEPROTECT) == 0) {
+            continue;
+        }
+        /* Remove protection and unregister all affected RAM blocks */
+        uffd_protect_memory(rs->uffdio_fd, (hwaddr) bs->host, bs->max_length, 
false);
+        uffd_unregister_memory(rs->uffdio_fd, (hwaddr) bs->host, 
bs->max_length);
+        /* Cleanup flags */
+        bs->flags &= ~RAM_UF_WRITEPROTECT;
+
+        info_report("UFFD-WP write-tracking disabled: "
+                "block_id=%s page_size=%zu start=%p length=%lu "
+                "romd_mode=%i ram=%i readonly=%i nonvolatile=%i rom_device=%i",
+                bs->idstr, bs->page_size, bs->host, bs->max_length,
+                bs->mr->romd_mode, bs->mr->ram, bs->mr->readonly,
+                bs->mr->nonvolatile, bs->mr->rom_device);
+    }
+
+    /* Finally close UFFD file descriptor */
+    uffd_close_fd(rs->uffdio_fd);
+    rs->uffdio_fd = -1;
+#else
+    error_setg(&migrate_get_current()->error,
+            "Background-snapshot not supported on non-Linux hosts");
+#endif /* CONFIG_LINUX */
+}
+
  static SaveVMHandlers savevm_ram_handlers = {
      .save_setup = ram_save_setup,
      .save_live_iterate = ram_save_iterate,
diff --git a/migration/ram.h b/migration/ram.h
index 011e85414e..0ec63e27ee 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -79,4 +79,8 @@ void colo_flush_ram_cache(void);
  void colo_release_ram_cache(void);
  void colo_incoming_start_dirty_log(void);

+/* Background snapshots */

+int ram_write_tracking_start(void);
+void ram_write_tracking_stop(void);
+
  #endif
diff --git a/util/meson.build b/util/meson.build
index f359af0d46..c64bfe94b3 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -50,6 +50,7 @@ endif

if have_system

    util_ss.add(when: 'CONFIG_GIO', if_true: [files('dbus.c'), gio])
+  util_ss.add(when: 'CONFIG_LINUX', if_true: files('userfaultfd.c'))
  endif

if have_block

diff --git a/util/userfaultfd.c b/util/userfaultfd.c
new file mode 100644
index 0000000000..038953d7ed
--- /dev/null
+++ b/util/userfaultfd.c
@@ -0,0 +1,215 @@
+/*
+ * Linux UFFD-WP support
+ *
+ * Copyright Virtuozzo GmbH, 2020
+ *
+ * Authors:
+ *  Andrey Gruzdev   <andrey.gruzdev@virtuozzo.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or
+ * later.  See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/bitops.h"
+#include "qemu/error-report.h"
+#include "qemu/userfaultfd.h"
+#include <poll.h>
+#include <sys/syscall.h>
+#include <sys/ioctl.h>
+
+/**
+ * uffd_create_fd: create UFFD file descriptor
+ *
+ * Returns non-negative file descriptor or negative value in case of an error
+ */
+int uffd_create_fd(void)
+{
+    int uffd;
+    struct uffdio_api api_struct;
+    uint64_t ioctl_mask = BIT(_UFFDIO_REGISTER) | BIT(_UFFDIO_UNREGISTER);
+
+    uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+    if (uffd < 0) {
+        error_report("uffd_create_fd() failed: UFFD not supported");


Please include the errno (or strerror(errno))

Ok.

+        return -1;
+    }
+
+    api_struct.api = UFFD_API;
+    api_struct.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
+    if (ioctl(uffd, UFFDIO_API, &api_struct)) {
+        error_report("uffd_create_fd() failed: "
+                     "API version not supported version=%llx errno=%i",
+                api_struct.api, errno);
+        goto fail;
+    }
+
+    if ((api_struct.ioctls & ioctl_mask) != ioctl_mask) {
+        error_report("uffd_create_fd() failed: "
+                     "PAGEFAULT_FLAG_WP feature missing");
+        goto fail;
+    }
+
+    return uffd;
+
+fail:
+    close(uffd);
+    return -1;
+}


It would be great if we could find a way to share this with
migration/postcopy-ram.c - this duplicates a lor of
request_ufd_features.

Now I'm changing userfaultfd.c glue code to support also the postcopyrequirenments - adding functionality to query UFFD feature support, gluefor COPY_PAGE/ZERO_PAGE IOCTLs etc.

+
+/**
+ * uffd_close_fd: close UFFD file descriptor
+ *
+ * @uffd: UFFD file descriptor
+ */
+void uffd_close_fd(int uffd)
+{
+    assert(uffd >= 0);
+    close(uffd);
+}
+
+/**
+ * uffd_register_memory: register memory range with UFFD
+ *
+ * Returns 0 in case of success, negative value on error
+ *
+ * @uffd: UFFD file descriptor
+ * @start: starting virtual address of memory range
+ * @length: length of memory range
+ * @track_missing: generate events on missing-page faults
+ * @track_wp: generate events on write-protected-page faults
+ */
+int uffd_register_memory(int uffd, hwaddr start, hwaddr length,
+        bool track_missing, bool track_wp)
+{
+    struct uffdio_register uffd_register;
+
+    uffd_register.range.start = start;
+    uffd_register.range.len = length;
+    uffd_register.mode = (track_missing ? UFFDIO_REGISTER_MODE_MISSING : 0) |
+                         (track_wp ? UFFDIO_REGISTER_MODE_WP : 0);
+
+    if (ioctl(uffd, UFFDIO_REGISTER, &uffd_register)) {
+        error_report("uffd_register_memory() failed: "
+                     "start=%0"PRIx64" len=%"PRIu64" mode=%llu errno=%i",


PRIx64 for mode as well.

Ok.

+                start, length, uffd_register.mode, errno);
+        return -1;
+    }
+
+    return 0;
+}
+
+/**
+ * uffd_unregister_memory: un-register memory range with UFFD
+ *
+ * Returns 0 in case of success, negative value on error
+ *
+ * @uffd: UFFD file descriptor
+ * @start: starting virtual address of memory range
+ * @length: length of memory range
+ */
+int uffd_unregister_memory(int uffd, hwaddr start, hwaddr length)
+{
+    struct uffdio_range uffd_range;
+
+    uffd_range.start = start;
+    uffd_range.len = length;
+
+    if (ioctl(uffd, UFFDIO_UNREGISTER, &uffd_range)) {
+        error_report("uffd_unregister_memory() failed: "
+                     "start=%0"PRIx64" len=%"PRIu64" errno=%i",
+                start, length, errno);
+        return -1;
+    }
+
+    return 0;
+}
+
+/**
+ * uffd_protect_memory: protect/unprotect memory range for writes with UFFD
+ *
+ * Returns 0 on success or negative value in case of error
+ *
+ * @uffd: UFFD file descriptor
+ * @start: starting virtual address of memory range
+ * @length: length of memory range
+ * @wp: write-protect/unprotect
+ */
+int uffd_protect_memory(int uffd, hwaddr start, hwaddr length, bool wp)
+{
+    struct uffdio_writeprotect uffd_writeprotect;
+    int res;
+
+    uffd_writeprotect.range.start = start;
+    uffd_writeprotect.range.len = length;
+    uffd_writeprotect.mode = (wp ? UFFDIO_WRITEPROTECT_MODE_WP : 0);
+
+    do {
+        res = ioctl(uffd, UFFDIO_WRITEPROTECT, &uffd_writeprotect);
+    } while (res < 0 && errno == EINTR);
+    if (res < 0) {
+        error_report("uffd_protect_memory() failed: "
+                     "start=%0"PRIx64" len=%"PRIu64" mode=%llu errno=%i",
+                start, length, uffd_writeprotect.mode, errno);
+        return -1;
+    }
+
+    return 0;
+}
+
+/**
+ * uffd_read_events: read pending UFFD events
+ *
+ * Returns number of fetched messages, 0 if non is available or
+ * negative value in case of an error
+ *
+ * @uffd: UFFD file descriptor
+ * @msgs: pointer to message buffer
+ * @count: number of messages that can fit in the buffer
+ */
+int uffd_read_events(int uffd, struct uffd_msg *msgs, int count)
+{
+    ssize_t res;
+    do {
+        res = read(uffd, msgs, count * sizeof(struct uffd_msg));
+    } while (res < 0 && errno == EINTR);
+
+    if ((res < 0 && errno == EAGAIN)) {
+        return 0;
+    }
+    if (res < 0) {
+        error_report("uffd_read_events() failed: errno=%i", errno);
+        return -1;
+    }
+
+    return (int) (res / sizeof(struct uffd_msg));
+}
+
+/**
+ * uffd_poll_events: poll UFFD file descriptor for read
+ *
+ * Returns true if events are available for read, false otherwise
+ *
+ * @uffd: UFFD file descriptor
+ * @tmo: timeout in milliseconds, 0 for non-blocking operation,
+ *       negative value for infinite wait
+ */
+bool uffd_poll_events(int uffd, int tmo)
+{
+    int res;
+    struct pollfd poll_fd = { .fd = uffd, .events = POLLIN, .revents = 0 };
+
+    do {
+        res = poll(&poll_fd, 1, tmo);
+    } while (res < 0 && errno == EINTR);
+
+    if (res == 0) {
+        return false;
+    }
+    if (res < 0) {
+        error_report("uffd_poll_events() failed: errno=%i", errno);
+        return false;
+    }
+
+    return (poll_fd.revents & POLLIN) != 0;
+}
--
2.25.1



--
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397
                virtuzzo.com

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