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

[Fam Zheng <famz@redhat.com>]

On Thu, 08/28 17:22, Benoît Canet wrote:
> 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 ?

Yeah, it has to absorb the "filename" option in null_file_open. The value would
be "null://" or "null-co://", and is parsed in generic open code.

> 
> > +        {
> > +            .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.

Good idea!

> 
> > +
> > +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 ?

Will do.

Fam