Re: [PULL 20/47] backends/iommufd: Introduce the iommufd object

["Cédric Le Goater" <clg@redhat.com>]

[ ... ]


> +static void iommufd_backend_init(Object *obj)
> +{
> +    IOMMUFDBackend *be = IOMMUFD_BACKEND(obj);
> +
> +    be->fd = -1;
> +    be->users = 0;
> +    be->owned = true;
> +    qemu_mutex_init(&be->lock);> +}
> +
> +static void iommufd_backend_finalize(Object *obj)
> +{
> +    IOMMUFDBackend *be = IOMMUFD_BACKEND(obj);
> +
> +    if (be->owned) {
> +        close(be->fd);
> +        be->fd = -1;
> +    }
> +}
> +
> +static void iommufd_backend_set_fd(Object *obj, const char *str, Error **errp)
> +{
> +    IOMMUFDBackend *be = IOMMUFD_BACKEND(obj);
> +    int fd = -1;
> +
> +    fd = monitor_fd_param(monitor_cur(), str, errp);
> +    if (fd == -1) {
> +        error_prepend(errp, "Could not parse remote object fd %s:", str);
> +        return;
> +    }
> +    qemu_mutex_lock(&be->lock);
> +    be->fd = fd;
> +    be->owned = false;
> +    qemu_mutex_unlock(&be->lock);
> +    trace_iommu_backend_set_fd(be->fd);
> +}
> +
> +static bool iommufd_backend_can_be_deleted(UserCreatable *uc)
> +{
> +    IOMMUFDBackend *be = IOMMUFD_BACKEND(uc);
> +
> +    return !be->users;

Coverity CID 1531549 reports a concurrent data access violation because
be->users is being accessed without holding the mutex.

I wonder how useful is this mutex anyhow, since the code paths should
be protected by the BQL lock. If you agree, I will send an update to
simply drop be->lock and solve this report.

Thanks,

C.



> +}
> +
> +static void iommufd_backend_class_init(ObjectClass *oc, void *data)
> +{
> +    UserCreatableClass *ucc = USER_CREATABLE_CLASS(oc);
> +
> +    ucc->can_be_deleted = iommufd_backend_can_be_deleted;
> +
> +    object_class_property_add_str(oc, "fd", NULL, iommufd_backend_set_fd);
> +}
> +
> +int iommufd_backend_connect(IOMMUFDBackend *be, Error **errp)
> +{
> +    int fd, ret = 0;
> +
> +    qemu_mutex_lock(&be->lock);
> +    if (be->users == UINT32_MAX) {
> +        error_setg(errp, "too many connections");
> +        ret = -E2BIG;
> +        goto out;
> +    }
> +    if (be->owned && !be->users) {
> +        fd = qemu_open_old("/dev/iommu", O_RDWR);
> +        if (fd < 0) {
> +            error_setg_errno(errp, errno, "/dev/iommu opening failed");
> +            ret = fd;
> +            goto out;
> +        }
> +        be->fd = fd;
> +    }
> +    be->users++;
> +out:
> +    trace_iommufd_backend_connect(be->fd, be->owned,
> +                                  be->users, ret);
> +    qemu_mutex_unlock(&be->lock);
> +    return ret;
> +}
> +
> +void iommufd_backend_disconnect(IOMMUFDBackend *be)
> +{
> +    qemu_mutex_lock(&be->lock);
> +    if (!be->users) {
> +        goto out;
> +    }
> +    be->users--;
> +    if (!be->users && be->owned) {
> +        close(be->fd);
> +        be->fd = -1;
> +    }
> +out:
> +    trace_iommufd_backend_disconnect(be->fd, be->users);
> +    qemu_mutex_unlock(&be->lock);
> +}
> +
> +int iommufd_backend_alloc_ioas(IOMMUFDBackend *be, uint32_t *ioas_id,
> +                               Error **errp)
> +{
> +    int ret, fd = be->fd;
> +    struct iommu_ioas_alloc alloc_data  = {
> +        .size = sizeof(alloc_data),
> +        .flags = 0,
> +    };
> +
> +    ret = ioctl(fd, IOMMU_IOAS_ALLOC, &alloc_data);
> +    if (ret) {
> +        error_setg_errno(errp, errno, "Failed to allocate ioas");
> +        return ret;
> +    }
> +
> +    *ioas_id = alloc_data.out_ioas_id;
> +    trace_iommufd_backend_alloc_ioas(fd, *ioas_id, ret);
> +
> +    return ret;
> +}
> +
> +void iommufd_backend_free_id(IOMMUFDBackend *be, uint32_t id)
> +{
> +    int ret, fd = be->fd;
> +    struct iommu_destroy des = {
> +        .size = sizeof(des),
> +        .id = id,
> +    };
> +
> +    ret = ioctl(fd, IOMMU_DESTROY, &des);
> +    trace_iommufd_backend_free_id(fd, id, ret);
> +    if (ret) {
> +        error_report("Failed to free id: %u %m", id);
> +    }
> +}
> +
> +int iommufd_backend_map_dma(IOMMUFDBackend *be, uint32_t ioas_id, hwaddr iova,
> +                            ram_addr_t size, void *vaddr, bool readonly)
> +{
> +    int ret, fd = be->fd;
> +    struct iommu_ioas_map map = {
> +        .size = sizeof(map),
> +        .flags = IOMMU_IOAS_MAP_READABLE |
> +                 IOMMU_IOAS_MAP_FIXED_IOVA,
> +        .ioas_id = ioas_id,
> +        .__reserved = 0,
> +        .user_va = (uintptr_t)vaddr,
> +        .iova = iova,
> +        .length = size,
> +    };
> +
> +    if (!readonly) {
> +        map.flags |= IOMMU_IOAS_MAP_WRITEABLE;
> +    }
> +
> +    ret = ioctl(fd, IOMMU_IOAS_MAP, &map);
> +    trace_iommufd_backend_map_dma(fd, ioas_id, iova, size,
> +                                  vaddr, readonly, ret);
> +    if (ret) {
> +        ret = -errno;
> +
> +        /* TODO: Not support mapping hardware PCI BAR region for now. */
> +        if (errno == EFAULT) {
> +            warn_report("IOMMU_IOAS_MAP failed: %m, PCI BAR?");
> +        } else {
> +            error_report("IOMMU_IOAS_MAP failed: %m");
> +        }
> +    }
> +    return ret;
> +}
> +
> +int iommufd_backend_unmap_dma(IOMMUFDBackend *be, uint32_t ioas_id,
> +                              hwaddr iova, ram_addr_t size)
> +{
> +    int ret, fd = be->fd;
> +    struct iommu_ioas_unmap unmap = {
> +        .size = sizeof(unmap),
> +        .ioas_id = ioas_id,
> +        .iova = iova,
> +        .length = size,
> +    };
> +
> +    ret = ioctl(fd, IOMMU_IOAS_UNMAP, &unmap);
> +    /*
> +     * IOMMUFD takes mapping as some kind of object, unmapping
> +     * nonexistent mapping is treated as deleting a nonexistent
> +     * object and return ENOENT. This is different from legacy
> +     * backend which allows it. vIOMMU may trigger a lot of
> +     * redundant unmapping, to avoid flush the log, treat them
> +     * as succeess for IOMMUFD just like legacy backend.
> +     */
> +    if (ret && errno == ENOENT) {
> +        trace_iommufd_backend_unmap_dma_non_exist(fd, ioas_id, iova, size, ret);
> +        ret = 0;
> +    } else {
> +        trace_iommufd_backend_unmap_dma(fd, ioas_id, iova, size, ret);
> +    }
> +
> +    if (ret) {
> +        ret = -errno;
> +        error_report("IOMMU_IOAS_UNMAP failed: %m");
> +    }
> +    return ret;
> +}
> +
> +static const TypeInfo iommufd_backend_info = {
> +    .name = TYPE_IOMMUFD_BACKEND,
> +    .parent = TYPE_OBJECT,
> +    .instance_size = sizeof(IOMMUFDBackend),
> +    .instance_init = iommufd_backend_init,
> +    .instance_finalize = iommufd_backend_finalize,
> +    .class_size = sizeof(IOMMUFDBackendClass),
> +    .class_init = iommufd_backend_class_init,
> +    .interfaces = (InterfaceInfo[]) {
> +        { TYPE_USER_CREATABLE },
> +        { }
> +    }
> +};
> +
> +static void register_types(void)
> +{
> +    type_register_static(&iommufd_backend_info);
> +}
> +
> +type_init(register_types);
> diff --git a/backends/Kconfig b/backends/Kconfig
> index f35abc16092808b1fe5b033a346908e2d66bff0b..2cb23f62fa1526cedafedcc99a032e098075b846 100644
> --- a/backends/Kconfig
> +++ b/backends/Kconfig
> @@ -1 +1,5 @@
>   source tpm/Kconfig
> +
> +config IOMMUFD
> +    bool
> +    depends on VFIO
> diff --git a/backends/meson.build b/backends/meson.build
> index 914c7c4afb905cfe710ad23dd1ee42907f6d1679..9a5cea480d172d50a641e4d9179093e8155f2db1 100644
> --- a/backends/meson.build
> +++ b/backends/meson.build
> @@ -20,6 +20,7 @@ if have_vhost_user
>     system_ss.add(when: 'CONFIG_VIRTIO', if_true: files('vhost-user.c'))
>   endif
>   system_ss.add(when: 'CONFIG_VIRTIO_CRYPTO', if_true: files('cryptodev-vhost.c'))
> +system_ss.add(when: 'CONFIG_IOMMUFD', if_true: files('iommufd.c'))
>   if have_vhost_user_crypto
>     system_ss.add(when: 'CONFIG_VIRTIO_CRYPTO', if_true: files('cryptodev-vhost-user.c'))
>   endif
> diff --git a/backends/trace-events b/backends/trace-events
> index 652eb76a5723e2053fe97338c481309c58284d6a..d45c6e31a67ed66d94787f60eb08a525cf6ff68b 100644
> --- a/backends/trace-events
> +++ b/backends/trace-events
> @@ -5,3 +5,13 @@ dbus_vmstate_pre_save(void)
>   dbus_vmstate_post_load(int version_id) "version_id: %d"
>   dbus_vmstate_loading(const char *id) "id: %s"
>   dbus_vmstate_saving(const char *id) "id: %s"
> +
> +# iommufd.c
> +iommufd_backend_connect(int fd, bool owned, uint32_t users, int ret) "fd=%d owned=%d users=%d (%d)"
> +iommufd_backend_disconnect(int fd, uint32_t users) "fd=%d users=%d"
> +iommu_backend_set_fd(int fd) "pre-opened /dev/iommu fd=%d"
> +iommufd_backend_map_dma(int iommufd, uint32_t ioas, uint64_t iova, uint64_t size, void *vaddr, bool readonly, int ret) " iommufd=%d ioas=%d iova=0x%"PRIx64" size=0x%"PRIx64" addr=%p readonly=%d (%d)"
> +iommufd_backend_unmap_dma_non_exist(int iommufd, uint32_t ioas, uint64_t iova, uint64_t size, int ret) " Unmap nonexistent mapping: iommufd=%d ioas=%d iova=0x%"PRIx64" size=0x%"PRIx64" (%d)"
> +iommufd_backend_unmap_dma(int iommufd, uint32_t ioas, uint64_t iova, uint64_t size, int ret) " iommufd=%d ioas=%d iova=0x%"PRIx64" size=0x%"PRIx64" (%d)"
> +iommufd_backend_alloc_ioas(int iommufd, uint32_t ioas, int ret) " iommufd=%d ioas=%d (%d)"
> +iommufd_backend_free_id(int iommufd, uint32_t id, int ret) " iommufd=%d id=%d (%d)"
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 42fd09e4de96e962cd5873c49501f6e1dbb5e346..5fe8ea57d2d2f9390a976ef2fefe86463e888bb1 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -5224,6 +5224,18 @@ SRST
>   
>           The ``share`` boolean option is on by default with memfd.
>   
> +    ``-object iommufd,id=id[,fd=fd]``
> +        Creates an iommufd backend which allows control of DMA mapping
> +        through the ``/dev/iommu`` device.
> +
> +        The ``id`` parameter is a unique ID which frontends (such as
> +        vfio-pci of vdpa) will use to connect with the iommufd backend.
> +
> +        The ``fd`` parameter is an optional pre-opened file descriptor
> +        resulting from ``/dev/iommu`` opening. Usually the iommufd is shared
> +        across all subsystems, bringing the benefit of centralized
> +        reference counting.
> +
>       ``-object rng-builtin,id=id``
>           Creates a random number generator backend which obtains entropy
>           from QEMU builtin functions. The ``id`` parameter is a unique ID