block: Introduce "null" driver

This is an analogue to Linux null_blk. It can be used for testing block
device emulation and general block layer functionalities such as
coroutines and throttling, where disk IO is not necessary or wanted.

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block/Makefile.objs |   1 +
 block/null.c        | 172 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 173 insertions(+)
 create mode 100644 block/null.c

On 08/27/2014 12:22 AM, Fam Zheng wrote:
> This is an analogue to Linux null_blk. It can be used for testing block
> device emulation and general block layer functionalities such as
> coroutines and throttling, where disk IO is not necessary or wanted.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/Makefile.objs |   1 +
>  block/null.c        | 172 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 173 insertions(+)
>  create mode 100644 block/null.c

Worth also adding it to BlockdevOptions in qapi/block-core.json to allow
hotplug of this driver?  (Not that libvirt will ever hotplug one, but I
really want to move towards being feature-complete on letting QMP drive
all block types)

Eric Blake <eblake@redhat.com> writes:

> On 08/27/2014 12:22 AM, Fam Zheng wrote:
>> This is an analogue to Linux null_blk. It can be used for testing block
>> device emulation and general block layer functionalities such as
>> coroutines and throttling, where disk IO is not necessary or wanted.
>> 
>> Signed-off-by: Fam Zheng <famz@redhat.com>
>> ---
>>  block/Makefile.objs |   1 +
>>  block/null.c | 172
>> ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 173 insertions(+)
>>  create mode 100644 block/null.c
>
> Worth also adding it to BlockdevOptions in qapi/block-core.json to allow
> hotplug of this driver?  (Not that libvirt will ever hotplug one, but I
> really want to move towards being feature-complete on letting QMP drive
> all block types)

I'd support a policy of "no new block drivers outside QAPI".

The Wednesday 27 Aug 2014 à 14:22:56 (+0800), Fam Zheng wrote :
> This is an analogue to Linux null_blk. It can be used for testing block
> device emulation and general block layer functionalities such as
> coroutines and throttling, where disk IO is not necessary or wanted.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/Makefile.objs |   1 +
>  block/null.c        | 172 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 173 insertions(+)
>  create mode 100644 block/null.c
> 
> diff --git a/block/Makefile.objs b/block/Makefile.objs
> index 858d2b3..087e281 100644
> --- a/block/Makefile.objs
> +++ b/block/Makefile.objs
> @@ -9,6 +9,7 @@ block-obj-y += snapshot.o qapi.o
>  block-obj-$(CONFIG_WIN32) += raw-win32.o win32-aio.o
>  block-obj-$(CONFIG_POSIX) += raw-posix.o
>  block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
> +block-obj-y += null.o
>  
>  ifeq ($(CONFIG_POSIX),y)
>  block-obj-y += nbd.o nbd-client.o sheepdog.o
> diff --git a/block/null.c b/block/null.c
> new file mode 100644
> index 0000000..d93a24d
> --- /dev/null
> +++ b/block/null.c
> @@ -0,0 +1,172 @@
> +/*
> + * Null block driver
> + *
> + * Authors:
> + *  Fam Zheng <famz@redhat.com>
> + *
> + * Copyright (C) 2014 Red Hat, Inc.
> + *
> + * 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 "block/block_int.h"
> +
> +typedef struct {
> +    int64_t length;
> +} BDRVNullState;
> +
> +static QemuOptsList runtime_opts = {
> +    .name = "null",
> +    .head = QTAILQ_HEAD_INITIALIZER(runtime_opts.head),
> +    .desc = {
> +        {
> +            .name = "filename",
> +            .type = QEMU_OPT_STRING,
> +            .help = "",
> +        },
> +        { /* end of list */ }
> +    },
> +};
> +
> +static int null_file_open(BlockDriverState *bs, QDict *options, int flags,
> +                          Error **errp)
> +{
> +    QemuOpts *opts;
> +
> +    opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);
> +    qemu_opts_absorb_qdict(opts, options, &error_abort);
> +    qemu_opts_del(opts);
> +    return 0;
> +}
> +
> +static void null_close(BlockDriverState *bs)
> +{
> +}
> +
> +static int64_t null_getlength(BlockDriverState *bs)
> +{
> +    /* Size is not very relevant, let's report 1G for now */
> +    return 1 << 30;
> +}
> +
> +#ifdef NULL_COROUTINE

Aren't you afraid that the conditional compilation will let parts of the code
bitrot ?

Best regards

Benoît

> +static coroutine_fn int null_co_read(BlockDriverState *bs, int64_t sector_num,
> +                                     uint8_t *buf, int nb_sectors)
> +{
> +    return 0;
> +}
> +
> +static coroutine_fn int null_co_write(BlockDriverState *bs, int64_t sector_num,
> +                                      const uint8_t *buf, int nb_sectors)
> +{
> +    return 0;
> +}
> +
> +static coroutine_fn int null_co_flush(BlockDriverState *bs)
> +{
> +    return 0;
> +}
> +#else
> +
> +typedef struct {
> +    BlockDriverAIOCB common;
> +    QEMUBH *bh;
> +} NullAIOCB;
> +
> +typedef struct BlkdebugSuspendedReq {
> +    Coroutine *co;
> +    char *tag;
> +    QLIST_ENTRY(BlkdebugSuspendedReq) next;
> +} BlkdebugSuspendedReq;
> +
> +static void null_aio_cancel(BlockDriverAIOCB *blockacb);
> +
> +static const AIOCBInfo null_aiocb_info = {
> +    .aiocb_size = sizeof(NullAIOCB),
> +    .cancel     = null_aio_cancel,
> +};
> +
> +static void null_bh_cb(void *opaque)
> +{
> +    NullAIOCB *acb = opaque;
> +    acb->common.cb(acb->common.opaque, 0);
> +    qemu_bh_delete(acb->bh);
> +    qemu_aio_release(acb);
> +}
> +
> +static BlockDriverAIOCB *null_aio_readv(BlockDriverState *bs,
> +                                        int64_t sector_num, QEMUIOVector *qiov,
> +                                        int nb_sectors,
> +                                        BlockDriverCompletionFunc *cb,
> +                                        void *opaque)
> +{
> +    NullAIOCB *acb;
> +
> +    acb = qemu_aio_get(&null_aiocb_info, bs, cb, opaque);
> +    acb->bh = aio_bh_new(bdrv_get_aio_context(bs), null_bh_cb, acb);
> +    qemu_bh_schedule(acb->bh);
> +    return &acb->common;
> +}
> +
> +static BlockDriverAIOCB *null_aio_writev(BlockDriverState *bs,
> +                                         int64_t sector_num, QEMUIOVector *qiov,
> +                                         int nb_sectors,
> +                                         BlockDriverCompletionFunc *cb,
> +                                         void *opaque)
> +{
> +    NullAIOCB *acb;
> +
> +    acb = qemu_aio_get(&null_aiocb_info, bs, cb, opaque);
> +    acb->bh = aio_bh_new(bdrv_get_aio_context(bs), null_bh_cb, acb);
> +    qemu_bh_schedule(acb->bh);
> +    return &acb->common;
> +}
> +
> +static BlockDriverAIOCB *null_aio_flush(BlockDriverState *bs,
> +                                        BlockDriverCompletionFunc *cb,
> +                                        void *opaque)
> +{
> +    NullAIOCB *acb;
> +
> +    acb = qemu_aio_get(&null_aiocb_info, bs, cb, opaque);
> +    acb->bh = aio_bh_new(bdrv_get_aio_context(bs), null_bh_cb, acb);
> +    qemu_bh_schedule(acb->bh);
> +    return &acb->common;
> +}
> +
> +static void null_aio_cancel(BlockDriverAIOCB *blockacb)
> +{
> +    NullAIOCB *acb = container_of(blockacb, NullAIOCB, common);
> +    qemu_bh_delete(acb->bh);
> +    qemu_aio_release(acb);
> +}
> +
> +#endif
> +
> +static BlockDriver bdrv_null = {
> +    .format_name            = "null",
> +    .protocol_name          = "null",
> +    .instance_size          = sizeof(BDRVNullState),
> +
> +    .bdrv_file_open         = null_file_open,
> +    .bdrv_close             = null_close,
> +    .bdrv_getlength         = null_getlength,
> +
> +#ifdef NULL_COROUTINE
> +    .bdrv_read              = null_co_read,
> +    .bdrv_write             = null_co_write,
> +    .bdrv_co_flush_to_disk  = null_co_flush,
> +#else
> +    .bdrv_aio_readv         = null_aio_readv,
> +    .bdrv_aio_writev        = null_aio_writev,
> +    .bdrv_aio_flush         = null_aio_flush,
> +#endif
> +};
> +
> +static void bdrv_null_init(void)
> +{
> +    bdrv_register(&bdrv_null);
> +}
> +
> +block_init(bdrv_null_init);
> -- 
> 2.1.0
> 
>