Re: [PATCH v3 1/5] vhost-user: Add VIRTIO Shared Memory map request

[Albert Esteve]

On Mon, Sep 16, 2024 at 7:21 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Thu, Sep 12, 2024 at 04:53:31PM +0200, Albert Esteve wrote:
> > Add SHMEM_MAP/UNMAP requests to vhost-user to
> > handle VIRTIO Shared Memory mappings.
> >
> > This request allows backends to dynamically map
> > fds into a VIRTIO Shared Memory Region indentified
> > by its `shmid`. Then, the fd memory is advertised
> > to the driver as a base addres + offset, so it
> > can be read/written (depending on the mmap flags
> > requested) while its valid.
> >
> > The backend can munmap the memory range
> > in a given VIRTIO Shared Memory Region (again,
> > identified by its `shmid`), to free it. Upon
> > receiving this message, the front-end must
> > mmap the regions with PROT_NONE to reserve
> > the virtual memory space.
> >
> > The device model needs to create MemoryRegion
> > instances for the VIRTIO Shared Memory Regions
> > and add them to the `VirtIODevice` instance.
> >
> > Signed-off-by: Albert Esteve <aesteve@redhat.com>
> > ---
> >  hw/virtio/vhost-user.c                    | 122 ++++++++++++++++++++++
> >  hw/virtio/virtio.c                        |  13 +++
> >  include/hw/virtio/virtio.h                |   5 +
> >  subprojects/libvhost-user/libvhost-user.c |  60 +++++++++++
> >  subprojects/libvhost-user/libvhost-user.h |  52 +++++++++
> >  5 files changed, 252 insertions(+)
> >
> > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > index 00561daa06..338cc942ec 100644
> > --- a/hw/virtio/vhost-user.c
> > +++ b/hw/virtio/vhost-user.c
> > @@ -115,6 +115,8 @@ typedef enum VhostUserBackendRequest {
> >      VHOST_USER_BACKEND_SHARED_OBJECT_ADD = 6,
> >      VHOST_USER_BACKEND_SHARED_OBJECT_REMOVE = 7,
> >      VHOST_USER_BACKEND_SHARED_OBJECT_LOOKUP = 8,
> > +    VHOST_USER_BACKEND_SHMEM_MAP = 9,
> > +    VHOST_USER_BACKEND_SHMEM_UNMAP = 10,
> >      VHOST_USER_BACKEND_MAX
> >  }  VhostUserBackendRequest;
> >
> > @@ -192,6 +194,24 @@ typedef struct VhostUserShared {
> >      unsigned char uuid[16];
> >  } VhostUserShared;
> >
> > +/* For the flags field of VhostUserMMap */
> > +#define VHOST_USER_FLAG_MAP_R (1u << 0)
> > +#define VHOST_USER_FLAG_MAP_W (1u << 1)
> > +
> > +typedef struct {
> > +    /* VIRTIO Shared Memory Region ID */
> > +    uint8_t shmid;
> > +    uint8_t padding[7];
> > +    /* File offset */
> > +    uint64_t fd_offset;
> > +    /* Offset within the VIRTIO Shared Memory Region */
> > +    uint64_t shm_offset;
> > +    /* Size of the mapping */
> > +    uint64_t len;
> > +    /* Flags for the mmap operation, from VHOST_USER_FLAG_* */
> > +    uint64_t flags;
> > +} VhostUserMMap;
> > +
> >  typedef struct {
> >      VhostUserRequest request;
> >
> > @@ -224,6 +244,7 @@ typedef union {
> >          VhostUserInflight inflight;
> >          VhostUserShared object;
> >          VhostUserTransferDeviceState transfer_state;
> > +        VhostUserMMap mmap;
> >  } VhostUserPayload;
> >
> >  typedef struct VhostUserMsg {
> > @@ -1749,6 +1770,100 @@ 
> > vhost_user_backend_handle_shared_object_lookup(struct vhost_user *u,
> >      return 0;
> >  }
> >
> > +static int
> > +vhost_user_backend_handle_shmem_map(struct vhost_dev *dev,
> > +                                    VhostUserMMap *vu_mmap,
> > +                                    int fd)
> > +{
> > +    void *addr = 0;
> > +    MemoryRegion *mr = NULL;
> > +
> > +    if (fd < 0) {
> > +        error_report("Bad fd for map");
> > +        return -EBADF;
> > +    }
> > +
> > +    if (!dev->vdev->shmem_list ||
> > +        dev->vdev->n_shmem_regions <= vu_mmap->shmid) {
> > +        error_report("Device only has %d VIRTIO Shared Memory Regions. "
> > +                     "Requested ID: %d",
> > +                     dev->vdev->n_shmem_regions, vu_mmap->shmid);
> > +        return -EFAULT;
> > +    }
> > +
> > +    mr = &dev->vdev->shmem_list[vu_mmap->shmid];
> > +
> > +    if (!mr) {
> > +        error_report("VIRTIO Shared Memory Region at "
> > +                     "ID %d unitialized", vu_mmap->shmid);
> > +        return -EFAULT;
> > +    }
> > +
> > +    if ((vu_mmap->shm_offset + vu_mmap->len) < vu_mmap->len ||
> > +        (vu_mmap->shm_offset + vu_mmap->len) > mr->size) {
> > +        error_report("Bad offset/len for mmap %" PRIx64 "+%" PRIx64,
> > +                     vu_mmap->shm_offset, vu_mmap->len);
> > +        return -EFAULT;
> > +    }
> > +
> > +    void *shmem_ptr = memory_region_get_ram_ptr(mr);
> > +
> > +    addr = mmap(shmem_ptr + vu_mmap->shm_offset, vu_mmap->len,
> > +        ((vu_mmap->flags & VHOST_USER_FLAG_MAP_R) ? PROT_READ : 0) |
> > +        ((vu_mmap->flags & VHOST_USER_FLAG_MAP_W) ? PROT_WRITE : 0),
> > +        MAP_SHARED | MAP_FIXED, fd, vu_mmap->fd_offset);
> > +
> > +    if (addr == MAP_FAILED) {
> > +        error_report("Failed to mmap mem fd");
> > +        return -EFAULT;
> > +    }
> > +
> > +    return 0;
> > +}
> > +
> > +static int
> > +vhost_user_backend_handle_shmem_unmap(struct vhost_dev *dev,
> > +                                      VhostUserMMap *vu_mmap)
> > +{
> > +    void *addr = 0;
> > +    MemoryRegion *mr = NULL;
> > +
> > +    if (!dev->vdev->shmem_list ||
> > +        dev->vdev->n_shmem_regions <= vu_mmap->shmid) {
> > +        error_report("Device only has %d VIRTIO Shared Memory Regions. "
> > +                     "Requested ID: %d",
> > +                     dev->vdev->n_shmem_regions, vu_mmap->shmid);
> > +        return -EFAULT;
> > +    }
> > +
> > +    mr = &dev->vdev->shmem_list[vu_mmap->shmid];
> > +
> > +    if (!mr) {
> > +        error_report("VIRTIO Shared Memory Region at "
> > +                     "ID %d unitialized", vu_mmap->shmid);
> > +        return -EFAULT;
> > +    }
> > +
> > +    if ((vu_mmap->shm_offset + vu_mmap->len) < vu_mmap->len ||
> > +        (vu_mmap->shm_offset + vu_mmap->len) > mr->size) {
> > +        error_report("Bad offset/len for mmap %" PRIx64 "+%" PRIx64,
> > +                     vu_mmap->shm_offset, vu_mmap->len);
> > +        return -EFAULT;
> > +    }
> > +
> > +    void *shmem_ptr = memory_region_get_ram_ptr(mr);
> > +
> > +    addr = mmap(shmem_ptr + vu_mmap->shm_offset, vu_mmap->len,
> > +                PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
> > +
> > +    if (addr == MAP_FAILED) {
> > +        error_report("Failed to unmap memory");
> > +        return -EFAULT;
> > +    }
> > +
> > +    return 0;
> > +}
> > +
> >  static void close_backend_channel(struct vhost_user *u)
> >  {
> >      g_source_destroy(u->backend_src);
> > @@ -1817,6 +1932,13 @@ static gboolean backend_read(QIOChannel *ioc, 
> > GIOCondition condition,
> >          ret = vhost_user_backend_handle_shared_object_lookup(dev->opaque, 
> > ioc,
> >                                                               &hdr, 
> > &payload);
> >          break;
> > +    case VHOST_USER_BACKEND_SHMEM_MAP:
> > +        ret = vhost_user_backend_handle_shmem_map(dev, &payload.mmap,
> > +                                                  fd ? fd[0] : -1);
> > +        break;
> > +    case VHOST_USER_BACKEND_SHMEM_UNMAP:
> > +        ret = vhost_user_backend_handle_shmem_unmap(dev, &payload.mmap);
> > +        break;
> >      default:
> >          error_report("Received unexpected msg type: %d.", hdr.request);
> >          ret = -EINVAL;
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 9e10cbc058..ccc4f2cd75 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -3059,6 +3059,17 @@ int virtio_save(VirtIODevice *vdev, QEMUFile *f)
> >      return vmstate_save_state(f, &vmstate_virtio, vdev, NULL);
> >  }
> >
> > +MemoryRegion *virtio_new_shmem_region(VirtIODevice *vdev)
> > +{
> > +    MemoryRegion *mr;
> > +    ++vdev->n_shmem_regions;
> > +    vdev->shmem_list = g_renew(MemoryRegion, vdev->shmem_list,
> > +                               vdev->n_shmem_regions);
> > +    mr = &vdev->shmem_list[vdev->n_shmem_regions - 1];
> > +    mr = g_new0(MemoryRegion, 1);
> > +    return mr;
> > +}
>
> This function looks broken. shmem_list[] is reallocated so old
> MemoryRegion pointers will be dangling pointers. And then the
> MemoryRegion is allocated again using g_new0() but there is no way to
> retrieve that address again via shmem_list[].
>
> I expected something like this:
>
>   MemoryRegion *virtio_new_shmem_region(VirtIODevice *vdev)
>   {
>       MemoryRegion *mr;
>
>       assert(vdev->n_shmem_regions < INT_MAX);
>       ++vdev->n_shmem_regions;
>       vdev->shmem_list = g_renew(MemoryRegion *, vdev->shmem_list,
>                                  vdev->n_shmem_regions);
>       mr = g_new0(MemoryRegion, 1);
>       vdev->shmem_list[vdev->n_shmem_regions - 1] = mr;
>       return mr;
>   }
>
> > +
> >  /* A wrapper for use as a VMState .put function */
> >  static int virtio_device_put(QEMUFile *f, void *opaque, size_t size,
> >                                const VMStateField *field, JSONWriter 
> > *vmdesc)
> > @@ -3481,6 +3492,8 @@ void virtio_init(VirtIODevice *vdev, uint16_t 
> > device_id, size_t config_size)
> >              virtio_vmstate_change, vdev);
> >      vdev->device_endian = virtio_default_endian();
> >      vdev->use_guest_notifier_mask = true;
> > +    vdev->shmem_list = NULL;
>
> shmem_list[] and each MemoryRegion needs to be free somewhere.
> virtio_device_instance_finalize()?
>
> > +    vdev->n_shmem_regions = 0;
> >  }
> >
> >  /*
> > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > index 0fcbc5c0c6..d4a2f664d9 100644
> > --- a/include/hw/virtio/virtio.h
> > +++ b/include/hw/virtio/virtio.h
> > @@ -167,6 +167,9 @@ struct VirtIODevice
> >       */
> >      EventNotifier config_notifier;
> >      bool device_iotlb_enabled;
> > +    /* Shared memory region for vhost-user mappings. */
> > +    MemoryRegion *shmem_list;
> > +    int n_shmem_regions;
> >  };
> >
> >  struct VirtioDeviceClass {
> > @@ -286,6 +289,8 @@ void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);
> >
> >  int virtio_save(VirtIODevice *vdev, QEMUFile *f);
> >
> > +MemoryRegion *virtio_new_shmem_region(VirtIODevice *vdev);
> > +
> >  extern const VMStateInfo virtio_vmstate_info;
> >
> >  #define VMSTATE_VIRTIO_DEVICE \
> > diff --git a/subprojects/libvhost-user/libvhost-user.c 
> > b/subprojects/libvhost-user/libvhost-user.c
> > index 9c630c2170..496268e12b 100644
> > --- a/subprojects/libvhost-user/libvhost-user.c
> > +++ b/subprojects/libvhost-user/libvhost-user.c
> > @@ -1592,6 +1592,66 @@ vu_rm_shared_object(VuDev *dev, unsigned char 
> > uuid[UUID_LEN])
> >      return vu_send_message(dev, &msg);
> >  }
> >
> > +bool
> > +vu_shmem_map(VuDev *dev, uint8_t shmid, uint64_t fd_offset,
> > +             uint64_t shm_offset, uint64_t len, uint64_t flags)
> > +{
> > +    VhostUserMsg vmsg = {
> > +        .request = VHOST_USER_BACKEND_SHMEM_MAP,
> > +        .size = sizeof(vmsg.payload.mmap),
> > +        .flags = VHOST_USER_VERSION,
> > +        .payload.mmap = {
> > +            .shmid = shmid,
> > +            .fd_offset = fd_offset,
> > +            .shm_offset = shm_offset,
> > +            .len = len,
> > +            .flags = flags,
> > +        },
> > +    };
> > +
> > +    if (vu_has_protocol_feature(dev, VHOST_USER_PROTOCOL_F_REPLY_ACK)) {
> > +        vmsg.flags |= VHOST_USER_NEED_REPLY_MASK;
> > +    }
> > +
> > +    pthread_mutex_lock(&dev->backend_mutex);
> > +    if (!vu_message_write(dev, dev->backend_fd, &vmsg)) {
> > +        pthread_mutex_unlock(&dev->backend_mutex);
> > +        return false;
> > +    }
> > +
> > +    /* Also unlocks the backend_mutex */
> > +    return vu_process_message_reply(dev, &vmsg);
> > +}
> > +
> > +bool
> > +vu_shmem_unmap(VuDev *dev, uint8_t shmid, uint64_t shm_offset, uint64_t 
> > len)
> > +{
> > +    VhostUserMsg vmsg = {
> > +        .request = VHOST_USER_BACKEND_SHMEM_UNMAP,
> > +        .size = sizeof(vmsg.payload.mmap),
> > +        .flags = VHOST_USER_VERSION,
> > +        .payload.mmap = {
> > +            .shmid = shmid,
> > +            .fd_offset = 0,
> > +            .shm_offset = shm_offset,
> > +            .len = len,
> > +        },
> > +    };
> > +
> > +    if (vu_has_protocol_feature(dev, VHOST_USER_PROTOCOL_F_REPLY_ACK)) {
> > +        vmsg.flags |= VHOST_USER_NEED_REPLY_MASK;
> > +    }
> > +
> > +    pthread_mutex_lock(&dev->backend_mutex);
> > +    if (!vu_message_write(dev, dev->backend_fd, &vmsg)) {
> > +        pthread_mutex_unlock(&dev->backend_mutex);
> > +        return false;
> > +    }
> > +
> > +    /* Also unlocks the backend_mutex */
> > +    return vu_process_message_reply(dev, &vmsg);
> > +}
> > +
> >  static bool
> >  vu_set_vring_call_exec(VuDev *dev, VhostUserMsg *vmsg)
> >  {
> > diff --git a/subprojects/libvhost-user/libvhost-user.h 
> > b/subprojects/libvhost-user/libvhost-user.h
> > index deb40e77b3..ea4902e876 100644
> > --- a/subprojects/libvhost-user/libvhost-user.h
> > +++ b/subprojects/libvhost-user/libvhost-user.h
> > @@ -127,6 +127,8 @@ typedef enum VhostUserBackendRequest {
> >      VHOST_USER_BACKEND_SHARED_OBJECT_ADD = 6,
> >      VHOST_USER_BACKEND_SHARED_OBJECT_REMOVE = 7,
> >      VHOST_USER_BACKEND_SHARED_OBJECT_LOOKUP = 8,
> > +    VHOST_USER_BACKEND_SHMEM_MAP = 9,
> > +    VHOST_USER_BACKEND_SHMEM_UNMAP = 10,
> >      VHOST_USER_BACKEND_MAX
> >  }  VhostUserBackendRequest;
> >
> > @@ -186,6 +188,24 @@ typedef struct VhostUserShared {
> >      unsigned char uuid[UUID_LEN];
> >  } VhostUserShared;
> >
> > +/* For the flags field of VhostUserMMap */
> > +#define VHOST_USER_FLAG_MAP_R (1u << 0)
> > +#define VHOST_USER_FLAG_MAP_W (1u << 1)
> > +
> > +typedef struct {
> > +    /* VIRTIO Shared Memory Region ID */
> > +    uint8_t shmid;
> > +    uint8_t padding[7];
> > +    /* File offset */
> > +    uint64_t fd_offset;
> > +    /* Offset within the VIRTIO Shared Memory Region */
> > +    uint64_t shm_offset;
> > +    /* Size of the mapping */
> > +    uint64_t len;
> > +    /* Flags for the mmap operation, from VHOST_USER_FLAG_* */
> > +    uint64_t flags;
> > +} VhostUserMMap;
> > +
> >  #if defined(_WIN32) && (defined(__x86_64__) || defined(__i386__))
> >  # define VU_PACKED __attribute__((gcc_struct, packed))
> >  #else
> > @@ -214,6 +234,7 @@ typedef struct VhostUserMsg {
> >          VhostUserVringArea area;
> >          VhostUserInflight inflight;
> >          VhostUserShared object;
> > +        VhostUserMMap mmap;
> >      } payload;
> >
> >      int fds[VHOST_MEMORY_BASELINE_NREGIONS];
> > @@ -597,6 +618,37 @@ bool vu_add_shared_object(VuDev *dev, unsigned char 
> > uuid[UUID_LEN]);
> >   */
> >  bool vu_rm_shared_object(VuDev *dev, unsigned char uuid[UUID_LEN]);
> >
> > +/**
> > + * vu_shmem_map:
> > + * @dev: a VuDev context
> > + * @shmid: VIRTIO Shared Memory Region ID
> > + * @fd_offset: File offset
> > + * @shm_offset: Offset within the VIRTIO Shared Memory Region
> > + * @len: Size of the mapping
> > + * @flags: Flags for the mmap operation
> > + *
> > + * Advertises a new mapping to be made in a given VIRTIO Shared Memory 
> > Region.
> > + *
> > + * Returns: TRUE on success, FALSE on failure.
> > + */
> > +bool vu_shmem_map(VuDev *dev, uint8_t shmid, uint64_t fd_offset,
> > +                  uint64_t shm_offset, uint64_t len, uint64_t flags);
>
> How is the fd passed to the front-end?

True, I have tested the backend part directly with rust-vmm
(https://github.com/rust-vmm/vhost/pull/251), so I have not run this
code myself.

I'll add an fd argument and try to cover it with a test at least, as
we discussed in other threads.

>
> > +
> > +/**
> > + * vu_shmem_map:
>
> "vu_shmem_unmap:"
>
> > + * @dev: a VuDev context
> > + * @shmid: VIRTIO Shared Memory Region ID
> > + * @fd_offset: File offset
> > + * @len: Size of the mapping
> > + *
> > + * The front-end un-mmaps a given range in the VIRTIO Shared Memory Region
> > + * with the requested `shmid`.
> > + *
> > + * Returns: TRUE on success, FALSE on failure.
> > + */
> > +bool vu_shmem_unmap(VuDev *dev, uint8_t shmid, uint64_t shm_offset,
> > +                    uint64_t len);
> > +
> >  /**
> >   * vu_queue_set_notification:
> >   * @dev: a VuDev context
> > --
> > 2.45.2
> >