Re: [Qemu-devel] [PATCH v3] block: Introduce "null" driver

["Benoît Canet" <benoit.canet@irqsave.net>]

The Thursday 28 Aug 2014 à 13:53:11 (+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.
> 
> Use null:// for AIO version, and null-co:// for coroutine version.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> 
> ---
> V3: Drop copy&paste blkdebug structure. (Beniot)
> 
> V2: Don't #ifdef code, add two drivers. (Benoit)
>     Add to QAPI BlockdevOptions. (Eric)
>     Add "file.size" option to override backend size. (What is a better
>     way to associate /dev/vd{a,b,c} with command line devices, if sizes
>     are the same?)
> ---
>  block/Makefile.objs  |   1 +
>  block/null.c         | 179 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  qapi/block-core.json |  19 +++++-
>  3 files changed, 197 insertions(+), 2 deletions(-)
>  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..171fd19
> --- /dev/null
> +++ b/block/null.c
> @@ -0,0 +1,179 @@
> +/*
> + * 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 = "",
> +        },

You seems to define filename both here and in QMP but null_file_open don't use it neither
any part of the code.

Do you declare it only to fool some other part of the block layer ?

> +        {
> +            .name = BLOCK_OPT_SIZE,
> +            .type = QEMU_OPT_SIZE,
> +            .help = "size of the null block",
> +        },
> +        { /* end of list */ }
> +    },
> +};
> +
> +static int null_file_open(BlockDriverState *bs, QDict *options, int flags,
> +                          Error **errp)
> +{
> +    QemuOpts *opts;
> +    BDRVNullState *s = bs->opaque;
> +
> +    opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);
> +    qemu_opts_absorb_qdict(opts, options, &error_abort);
> +    s->length =
> +        qemu_opt_get_size(opts, BLOCK_OPT_SIZE, 1 << 30);
> +    qemu_opts_del(opts);
> +    return 0;
> +}
> +
> +static void null_close(BlockDriverState *bs)
> +{
> +}
> +
> +static int64_t null_getlength(BlockDriverState *bs)
> +{
> +    BDRVNullState *s = bs->opaque;
> +    return s->length;
> +}
> +
> +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;
> +}
> +
> +typedef struct {
> +    BlockDriverAIOCB common;
> +    QEMUBH *bh;
> +} NullAIOCB;
> +
> +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;
> +}

The former three function body are strictly identical maybe you could factorize them.
They would become thin wrapper around one common function.

> +
> +static void null_aio_cancel(BlockDriverAIOCB *blockacb)
> +{
> +    NullAIOCB *acb = container_of(blockacb, NullAIOCB, common);
> +    qemu_bh_delete(acb->bh);
> +    qemu_aio_release(acb);
> +}
> +
> +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,
> +
> +    .bdrv_aio_readv         = null_aio_readv,
> +    .bdrv_aio_writev        = null_aio_writev,
> +    .bdrv_aio_flush         = null_aio_flush,
> +};
> +
> +static BlockDriver bdrv_null_co = {
> +    .format_name            = "null-co",
> +    .protocol_name          = "null-co",
> +    .instance_size          = sizeof(BDRVNullState),
> +
> +    .bdrv_file_open         = null_file_open,
> +    .bdrv_close             = null_close,
> +    .bdrv_getlength         = null_getlength,
> +
> +    .bdrv_read              = null_co_read,
> +    .bdrv_write             = null_co_write,
> +    .bdrv_co_flush_to_disk  = null_co_flush,
> +};
> +
> +static void bdrv_null_init(void)
> +{
> +    bdrv_register(&bdrv_null);
> +    bdrv_register(&bdrv_null_co);
> +}
> +
> +block_init(bdrv_null_init);
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index fb74c56..9b2c9c6 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1150,7 +1150,8 @@
>    'data': [ 'archipelago', 'file', 'host_device', 'host_cdrom', 'host_floppy',
>              'http', 'https', 'ftp', 'ftps', 'tftp', 'vvfat', 'blkdebug',
>              'blkverify', 'bochs', 'cloop', 'cow', 'dmg', 'parallels', 'qcow',
> -            'qcow2', 'qed', 'raw', 'vdi', 'vhdx', 'vmdk', 'vpc', 'quorum' ] }
> +            'qcow2', 'qed', 'raw', 'vdi', 'vhdx', 'vmdk', 'vpc', 'quorum',
> +            'null' ] }

Why not also adding null-co to QMP ?

>  
>  ##
>  # @BlockdevOptionsBase
> @@ -1203,6 +1204,19 @@
>    'data': { 'filename': 'str' } }
>  
>  ##
> +# @BlockdevOptionsNull
> +#
> +# Driver specific block device options for the null backend.
> +#
> +# @size:    size of the device in bytes.
> +#
> +# Since: 2.2
> +##
> +{ 'type': 'BlockdevOptionsNull',
> +  'base': 'BlockdevOptionsFile',
> +  'data': { '*size': 'int' } }
> +
> +##
>  # @BlockdevOptionsVVFAT
>  #
>  # Driver specific block device options for the vvfat protocol.
> @@ -1484,7 +1498,8 @@
>        'vhdx':       'BlockdevOptionsGenericFormat',
>        'vmdk':       'BlockdevOptionsGenericCOWFormat',
>        'vpc':        'BlockdevOptionsGenericFormat',
> -      'quorum':     'BlockdevOptionsQuorum'
> +      'quorum':     'BlockdevOptionsQuorum',
> +      'null':       'BlockdevOptionsNull'
>    } }
>  
>  ##
> -- 
> 2.1.0
>