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

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

* 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

> +{
> +#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)) 

> +        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.

> +
> +/**
> + * 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.

> +                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
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK