Re: [RFC v3 1/8] blkio: add io_uring block driver using libblkio

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Jul 12, 2022 at 04:23:32PM +0200, Stefano Garzarella wrote:
> On Fri, Jul 08, 2022 at 05:17:30AM +0100, Stefan Hajnoczi wrote:
> > libblkio (https://gitlab.com/libblkio/libblkio/) is a library for
> > high-performance disk I/O. It currently supports io_uring and
> > virtio-blk-vhost-vdpa with additional drivers under development.
> > 
> > One of the reasons for developing libblkio is that other applications
> > besides QEMU can use it. This will be particularly useful for
> > vhost-user-blk which applications may wish to use for connecting to
> > qemu-storage-daemon.
> > 
> > libblkio also gives us an opportunity to develop in Rust behind a C API
> > that is easy to consume from QEMU.
> > 
> > This commit adds io_uring and virtio-blk-vhost-vdpa BlockDrivers to QEMU
> > using libblkio. It will be easy to add other libblkio drivers since they
> > will share the majority of code.
> > 
> > For now I/O buffers are copied through bounce buffers if the libblkio
> > driver requires it. Later commits add an optimization for
> > pre-registering guest RAM to avoid bounce buffers.
> > 
> > The syntax is:
> > 
> >  --blockdev io_uring,node-name=drive0,filename=test.img,readonly=on|off,cache.direct=on|off
> > 
> > and:
> > 
> >  --blockdev virtio-blk-vhost-vdpa,node-name=drive0,path=/dev/vdpa...,readonly=on|off
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > MAINTAINERS                   |   6 +
> > meson_options.txt             |   2 +
> > qapi/block-core.json          |  37 +-
> > meson.build                   |   9 +
> > block/blkio.c                 | 659 ++++++++++++++++++++++++++++++++++
> > tests/qtest/modules-test.c    |   3 +
> > block/meson.build             |   1 +
> > scripts/meson-buildoptions.sh |   3 +
> > 8 files changed, 718 insertions(+), 2 deletions(-)
> > create mode 100644 block/blkio.c
> > 
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 450abd0252..50f340d9ee 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -3395,6 +3395,12 @@ L: qemu-block@nongnu.org
> > S: Maintained
> > F: block/vdi.c
> > 
> > +blkio
> > +M: Stefan Hajnoczi <stefanha@redhat.com>
> > +L: qemu-block@nongnu.org
> > +S: Maintained
> > +F: block/blkio.c
> > +
> > iSCSI
> > M: Ronnie Sahlberg <ronniesahlberg@gmail.com>
> > M: Paolo Bonzini <pbonzini@redhat.com>
> > diff --git a/meson_options.txt b/meson_options.txt
> > index 97c38109b1..b0b2e0c9b5 100644
> > --- a/meson_options.txt
> > +++ b/meson_options.txt
> > @@ -117,6 +117,8 @@ option('bzip2', type : 'feature', value : 'auto',
> >        description: 'bzip2 support for DMG images')
> > option('cap_ng', type : 'feature', value : 'auto',
> >        description: 'cap_ng support')
> > +option('blkio', type : 'feature', value : 'auto',
> > +       description: 'libblkio block device driver')
> > option('bpf', type : 'feature', value : 'auto',
> >         description: 'eBPF support')
> > option('cocoa', type : 'feature', value : 'auto',
> > diff --git a/qapi/block-core.json b/qapi/block-core.json
> > index 2173e7734a..aa63d5e9bd 100644
> > --- a/qapi/block-core.json
> > +++ b/qapi/block-core.json
> > @@ -2951,11 +2951,15 @@
> >             'file', 'snapshot-access', 'ftp', 'ftps', 'gluster',
> >             {'name': 'host_cdrom', 'if': 'HAVE_HOST_BLOCK_DEVICE' },
> >             {'name': 'host_device', 'if': 'HAVE_HOST_BLOCK_DEVICE' },
> > -            'http', 'https', 'iscsi',
> > +            'http', 'https',
> > +            { 'name': 'io_uring', 'if': 'CONFIG_BLKIO' },
> > +            'iscsi',
> >             'luks', 'nbd', 'nfs', 'null-aio', 'null-co', 'nvme', 'parallels',
> >             'preallocate', 'qcow', 'qcow2', 'qed', 'quorum', 'raw', 'rbd',
> >             { 'name': 'replication', 'if': 'CONFIG_REPLICATION' },
> > -            'ssh', 'throttle', 'vdi', 'vhdx', 'vmdk', 'vpc', 'vvfat' ] }
> > +            'ssh', 'throttle', 'vdi', 'vhdx',
> > +            { 'name': 'virtio-blk-vhost-vdpa', 'if': 'CONFIG_BLKIO' },
> > +            'vmdk', 'vpc', 'vvfat' ] }
> > 
> > ##
> > # @BlockdevOptionsFile:
> > @@ -3678,6 +3682,30 @@
> >             '*debug': 'int',
> >             '*logfile': 'str' } }
> > 
> > +##
> > +# @BlockdevOptionsIoUring:
> > +#
> > +# Driver specific block device options for the io_uring backend.
> > +#
> > +# @filename: path to the image file
> > +#
> > +# Since: 7.1
> > +##
> > +{ 'struct': 'BlockdevOptionsIoUring',
> > +  'data': { 'filename': 'str' } }
> > +
> > +##
> > +# @BlockdevOptionsVirtioBlkVhostVdpa:
> > +#
> > +# Driver specific block device options for the virtio-blk-vhost-vdpa backend.
> > +#
> > +# @path: path to the vhost-vdpa character device.
> > +#
> > +# Since: 7.1
> > +##
> > +{ 'struct': 'BlockdevOptionsVirtioBlkVhostVdpa',
> > +  'data': { 'path': 'str' } }
> > +
> > ##
> > # @IscsiTransport:
> > #
> > @@ -4305,6 +4333,8 @@
> >                        'if': 'HAVE_HOST_BLOCK_DEVICE' },
> >       'http':       'BlockdevOptionsCurlHttp',
> >       'https':      'BlockdevOptionsCurlHttps',
> > +      'io_uring':   { 'type': 'BlockdevOptionsIoUring',
> > +                      'if': 'CONFIG_BLKIO' },
> >       'iscsi':      'BlockdevOptionsIscsi',
> >       'luks':       'BlockdevOptionsLUKS',
> >       'nbd':        'BlockdevOptionsNbd',
> > @@ -4327,6 +4357,9 @@
> >       'throttle':   'BlockdevOptionsThrottle',
> >       'vdi':        'BlockdevOptionsGenericFormat',
> >       'vhdx':       'BlockdevOptionsGenericFormat',
> > +      'virtio-blk-vhost-vdpa':
> > +                    { 'type': 'BlockdevOptionsVirtioBlkVhostVdpa',
> > +                      'if': 'CONFIG_BLKIO' },
> >       'vmdk':       'BlockdevOptionsGenericCOWFormat',
> >       'vpc':        'BlockdevOptionsGenericFormat',
> >       'vvfat':      'BlockdevOptionsVVFAT'
> > diff --git a/meson.build b/meson.build
> > index bc5569ace1..f09b009428 100644
> > --- a/meson.build
> > +++ b/meson.build
> > @@ -713,6 +713,13 @@ if not get_option('virglrenderer').auto() or have_system or have_vhost_user_gpu
> >                      required: get_option('virglrenderer'),
> >                      kwargs: static_kwargs)
> > endif
> > +blkio = not_found
> > +if not get_option('blkio').auto() or have_block
> > +  blkio = dependency('blkio',
> > +                     method: 'pkg-config',
> > +                     required: get_option('blkio'),
> > +                     kwargs: static_kwargs)
> > +endif
> > curl = not_found
> > if not get_option('curl').auto() or have_block
> >   curl = dependency('libcurl', version: '>=7.29.0',
> > @@ -1755,6 +1762,7 @@ config_host_data.set('CONFIG_LIBUDEV', libudev.found())
> > config_host_data.set('CONFIG_LZO', lzo.found())
> > config_host_data.set('CONFIG_MPATH', mpathpersist.found())
> > config_host_data.set('CONFIG_MPATH_NEW_API', mpathpersist_new_api)
> > +config_host_data.set('CONFIG_BLKIO', blkio.found())
> > config_host_data.set('CONFIG_CURL', curl.found())
> > config_host_data.set('CONFIG_CURSES', curses.found())
> > config_host_data.set('CONFIG_GBM', gbm.found())
> > @@ -3909,6 +3917,7 @@ summary_info += {'PAM':               pam}
> > summary_info += {'iconv support':     iconv}
> > summary_info += {'curses support':    curses}
> > summary_info += {'virgl support':     virgl}
> > +summary_info += {'blkio support':     blkio}
> > summary_info += {'curl support':      curl}
> > summary_info += {'Multipath support': mpathpersist}
> > summary_info += {'PNG support':       png}
> > diff --git a/block/blkio.c b/block/blkio.c
> > new file mode 100644
> > index 0000000000..7fbdbd7fae
> > --- /dev/null
> > +++ b/block/blkio.c
> > @@ -0,0 +1,659 @@
> > +#include "qemu/osdep.h"
> > +#include <blkio.h>
> > +#include "block/block_int.h"
> > +#include "qapi/error.h"
> > +#include "qapi/qmp/qdict.h"
> > +#include "qemu/module.h"
> > +
> > +typedef struct BlkAIOCB {
> > +    BlockAIOCB common;
> > +    struct blkio_mem_region mem_region;
> > +    QEMUIOVector qiov;
> > +    struct iovec bounce_iov;
> > +} BlkioAIOCB;
> > +
> > +typedef struct {
> > +    /* Protects ->blkio and request submission on ->blkioq */
> > +    QemuMutex lock;
> > +
> > +    struct blkio *blkio;
> > +    struct blkioq *blkioq; /* this could be multi-queue in the future */
> > +    int completion_fd;
> > +
> > +    /* Polling fetches the next completion into this field */
> > +    struct blkio_completion poll_completion;
> > +
> > +    /* The value of the "mem-region-alignment" property */
> > +    size_t mem_region_alignment;
> > +
> > +    /* Can we skip adding/deleting blkio_mem_regions? */
> > +    bool needs_mem_regions;
> > +} BDRVBlkioState;
> > +
> > +static void blkio_aiocb_complete(BlkioAIOCB *acb, int ret)
> > +{
> > +    /* Copy bounce buffer back to qiov */
> > +    if (acb->qiov.niov > 0) {
> > +        qemu_iovec_from_buf(&acb->qiov, 0,
> > +                acb->bounce_iov.iov_base,
> > +                acb->bounce_iov.iov_len);
> > +        qemu_iovec_destroy(&acb->qiov);
> > +    }
> > +
> > +    acb->common.cb(acb->common.opaque, ret);
> > +
> > +    if (acb->mem_region.len > 0) {
> > +        BDRVBlkioState *s = acb->common.bs->opaque;
> > +
> > +        WITH_QEMU_LOCK_GUARD(&s->lock) {
> > +            blkio_free_mem_region(s->blkio, &acb->mem_region);
> > +        }
> > +    }
> > +
> > +    qemu_aio_unref(&acb->common);
> > +}
> > +
> > +/*
> > + * Only the thread that calls aio_poll() invokes fd and poll handlers.
> > + * Therefore locks are not necessary except when accessing s->blkio.
> > + *
> > + * No locking is performed around blkioq_get_completions() although other
> > + * threads may submit I/O requests on s->blkioq. We're assuming there is no
> > + * inteference between blkioq_get_completions() and other s->blkioq APIs.
> > + */
> > +
> > +static void blkio_completion_fd_read(void *opaque)
> > +{
> > +    BlockDriverState *bs = opaque;
> > +    BDRVBlkioState *s = bs->opaque;
> > +    struct blkio_completion completion;
> > +    uint64_t val;
> > +    ssize_t ret __attribute__((unused));
> > +
> > +    /* Polling may have already fetched a completion */
> > +    if (s->poll_completion.user_data != NULL) {
> > +        completion = s->poll_completion;
> > +
> > +        /* Clear it in case blkio_aiocb_complete() has a nested event loop */
> > +        s->poll_completion.user_data = NULL;
> > +
> > +        blkio_aiocb_complete(completion.user_data, completion.ret);
> > +    }
> > +
> > +    /* Reset completion fd status */
> > +    ret = read(s->completion_fd, &val, sizeof(val));
> > +
> > +    /*
> > +     * Reading one completion at a time makes nested event loop re-entrancy
> > +     * simple. Change this loop to get multiple completions in one go if it
> > +     * becomes a performance bottleneck.
> > +     */
> > +    while (blkioq_do_io(s->blkioq, &completion, 0, 1, NULL) == 1) {
> > +        blkio_aiocb_complete(completion.user_data, completion.ret);
> > +    }
> > +}
> > +
> > +static bool blkio_completion_fd_poll(void *opaque)
> > +{
> > +    BlockDriverState *bs = opaque;
> > +    BDRVBlkioState *s = bs->opaque;
> > +
> > +    /* Just in case we already fetched a completion */
> > +    if (s->poll_completion.user_data != NULL) {
> > +        return true;
> > +    }
> > +
> > +    return blkioq_do_io(s->blkioq, &s->poll_completion, 0, 1, NULL) == 1;
> > +}
> > +
> > +static void blkio_completion_fd_poll_ready(void *opaque)
> > +{
> > +    blkio_completion_fd_read(opaque);
> > +}
> > +
> > +static void blkio_attach_aio_context(BlockDriverState *bs,
> > +                                     AioContext *new_context)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +
> > +    aio_set_fd_handler(new_context,
> > +                       s->completion_fd,
> > +                       false,
> > +                       blkio_completion_fd_read,
> > +                       NULL,
> > +                       blkio_completion_fd_poll,
> > +                       blkio_completion_fd_poll_ready,
> > +                       bs);
> > +}
> > +
> > +static void blkio_detach_aio_context(BlockDriverState *bs)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +
> > +    aio_set_fd_handler(bdrv_get_aio_context(bs),
> > +                       s->completion_fd,
> > +                       false, NULL, NULL, NULL, NULL, NULL);
> > +}
> > +
> > +static const AIOCBInfo blkio_aiocb_info = {
> > +    .aiocb_size = sizeof(BlkioAIOCB),
> > +};
> > +
> > +/* Create a BlkioAIOCB */
> > +static BlkioAIOCB *blkio_aiocb_get(BlockDriverState *bs,
> > +                                   BlockCompletionFunc *cb,
> > +                                   void *opaque)
> > +{
> > +    BlkioAIOCB *acb = qemu_aio_get(&blkio_aiocb_info, bs, cb, opaque);
> > +
> > +    /* A few fields need to be initialized, leave the rest... */
> > +    acb->qiov.niov = 0;
> > +    acb->mem_region.len = 0;
> > +    return acb;
> > +}
> > +
> > +/* s->lock must be held */
> > +static int blkio_aiocb_init_mem_region_locked(BlkioAIOCB *acb, size_t len)
> > +{
> > +    BDRVBlkioState *s = acb->common.bs->opaque;
> > +    size_t mem_region_len = QEMU_ALIGN_UP(len, s->mem_region_alignment);
> > +    int ret;
> > +
> > +    ret = blkio_alloc_mem_region(s->blkio, &acb->mem_region, mem_region_len);
> > +    if (ret < 0) {
> > +        return ret;
> > +    }
> > +
> > +    acb->bounce_iov.iov_base = acb->mem_region.addr;
> > +    acb->bounce_iov.iov_len = len;
> > +    return 0;
> > +}
> > +
> > +/* Call this to submit I/O after enqueuing a new request */
> > +static void blkio_submit_io(BlockDriverState *bs)
> > +{
> > +    if (qatomic_read(&bs->io_plugged) == 0) {
> > +        BDRVBlkioState *s = bs->opaque;
> > +
> > +        blkioq_do_io(s->blkioq, NULL, 0, 0, NULL);
> > +    }
> > +}
> > +
> > +static BlockAIOCB *blkio_aio_pdiscard(BlockDriverState *bs, int64_t offset,
> > +        int bytes, BlockCompletionFunc *cb, void *opaque)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +    BlkioAIOCB *acb;
> > +
> > +    QEMU_LOCK_GUARD(&s->lock);
> > +
> > +    acb = blkio_aiocb_get(bs, cb, opaque);
> > +    blkioq_discard(s->blkioq, offset, bytes, acb, 0);
> > +    blkio_submit_io(bs);
> > +    return &acb->common;
> > +}
> > +
> > +static BlockAIOCB *blkio_aio_preadv(BlockDriverState *bs, int64_t offset,
> > +        int64_t bytes, QEMUIOVector *qiov, BdrvRequestFlags flags,
> > +        BlockCompletionFunc *cb, void *opaque)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +    struct iovec *iov = qiov->iov;
> > +    int iovcnt = qiov->niov;
> > +    BlkioAIOCB *acb;
> > +
> > +    QEMU_LOCK_GUARD(&s->lock);
> > +
> > +    acb = blkio_aiocb_get(bs, cb, opaque);
> > +
> > +    if (s->needs_mem_regions) {
> > +        if (blkio_aiocb_init_mem_region_locked(acb, bytes) < 0) {
> > +            qemu_aio_unref(&acb->common);
> > +            return NULL;
> > +        }
> > +
> > +        /* Copy qiov because we'll call qemu_iovec_from_buf() on completion */
> > +        qemu_iovec_init_slice(&acb->qiov, qiov, 0, qiov->size);
> > +
> > +        iov = &acb->bounce_iov;
> > +        iovcnt = 1;
> > +    }
> > +
> > +    blkioq_readv(s->blkioq, offset, iov, iovcnt, acb, 0);
> > +    blkio_submit_io(bs);
> > +    return &acb->common;
> > +}
> > +
> > +static BlockAIOCB *blkio_aio_pwritev(BlockDriverState *bs, int64_t offset,
> > +        int64_t bytes, QEMUIOVector *qiov, BdrvRequestFlags flags,
> > +        BlockCompletionFunc *cb, void *opaque)
> > +{
> > +    uint32_t blkio_flags = (flags & BDRV_REQ_FUA) ? BLKIO_REQ_FUA : 0;
> > +    BDRVBlkioState *s = bs->opaque;
> > +    struct iovec *iov = qiov->iov;
> > +    int iovcnt = qiov->niov;
> > +    BlkioAIOCB *acb;
> > +
> > +    QEMU_LOCK_GUARD(&s->lock);
> > +
> > +    acb = blkio_aiocb_get(bs, cb, opaque);
> > +
> > +    if (s->needs_mem_regions) {
> > +        if (blkio_aiocb_init_mem_region_locked(acb, bytes) < 0) {
> > +            qemu_aio_unref(&acb->common);
> > +            return NULL;
> > +        }
> > +
> > +        qemu_iovec_to_buf(qiov, 0, acb->bounce_iov.iov_base, bytes);
> > +
> > +        iov = &acb->bounce_iov;
> > +        iovcnt = 1;
> > +    }
> > +
> > +    blkioq_writev(s->blkioq, offset, iov, iovcnt, acb, blkio_flags);
> > +    blkio_submit_io(bs);
> > +    return &acb->common;
> > +}
> > +
> > +static BlockAIOCB *blkio_aio_flush(BlockDriverState *bs,
> > +                                   BlockCompletionFunc *cb,
> > +                                   void *opaque)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +    BlkioAIOCB *acb;
> > +
> > +    QEMU_LOCK_GUARD(&s->lock);
> > +
> > +    acb = blkio_aiocb_get(bs, cb, opaque);
> > +
> > +    blkioq_flush(s->blkioq, acb, 0);
> > +    blkio_submit_io(bs);
> > +    return &acb->common;
> > +}
> > +
> > +/* For async to .bdrv_co_*() conversion */
> > +typedef struct {
> > +    Coroutine *coroutine;
> > +    int ret;
> > +} BlkioCoData;
> > +
> > +static void blkio_co_pwrite_zeroes_complete(void *opaque, int ret)
> > +{
> > +    BlkioCoData *data = opaque;
> > +
> > +    data->ret = ret;
> > +    aio_co_wake(data->coroutine);
> > +}
> > +
> > +static int coroutine_fn blkio_co_pwrite_zeroes(BlockDriverState *bs,
> > +    int64_t offset, int64_t bytes, BdrvRequestFlags flags)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +    BlkioCoData data = {
> > +        .coroutine = qemu_coroutine_self(),
> > +    };
> > +    uint32_t blkio_flags = 0;
> > +
> > +    if (flags & BDRV_REQ_FUA) {
> > +        blkio_flags |= BLKIO_REQ_FUA;
> > +    }
> > +    if (!(flags & BDRV_REQ_MAY_UNMAP)) {
> > +        blkio_flags |= BLKIO_REQ_NO_UNMAP;
> > +    }
> > +    if (flags & BDRV_REQ_NO_FALLBACK) {
> > +        blkio_flags |= BLKIO_REQ_NO_FALLBACK;
> > +    }
> > +
> > +    WITH_QEMU_LOCK_GUARD(&s->lock) {
> > +        BlkioAIOCB *acb =
> > +            blkio_aiocb_get(bs, blkio_co_pwrite_zeroes_complete, &data);
> > +        blkioq_write_zeroes(s->blkioq, offset, bytes, acb, blkio_flags);
> > +        blkio_submit_io(bs);
> > +    }
> > +
> > +    qemu_coroutine_yield();
> > +    return data.ret;
> > +}
> > +
> > +static void blkio_io_unplug(BlockDriverState *bs)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +
> > +    WITH_QEMU_LOCK_GUARD(&s->lock) {
> > +        blkio_submit_io(bs);
> > +    }
> > +}
> > +
> > +static void blkio_parse_filename_io_uring(const char *filename, QDict *options,
> > +                                          Error **errp)
> > +{
> > +    bdrv_parse_filename_strip_prefix(filename, "io_uring:", options);
> > +}
> > +
> > +static void blkio_parse_filename_virtio_blk_vhost_vdpa(
> > +        const char *filename,
> > +        QDict *options,
> > +        Error **errp)
> > +{
> > +    bdrv_parse_filename_strip_prefix(filename, "virtio-blk-vhost-vdpa:", options);
> > +}
> > +
> > +static int blkio_io_uring_open(BlockDriverState *bs, QDict *options, int flags,
> > +                               Error **errp)
> > +{
> > +    const char *filename = qdict_get_try_str(options, "filename");
> > +    BDRVBlkioState *s = bs->opaque;
> > +    int ret;
> > +
> > +    ret = blkio_set_str(s->blkio, "path", filename);
> > +    qdict_del(options, "filename");
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "failed to set path: %s",
> > +                         blkio_get_error_msg());
> > +        return ret;
> > +    }
> > +
> > +    if (flags & BDRV_O_NOCACHE) {
> > +        ret = blkio_set_bool(s->blkio, "direct", true);
> > +        if (ret < 0) {
> > +            error_setg_errno(errp, -ret, "failed to set direct: %s",
> > +                             blkio_get_error_msg());
> > +            return ret;
> > +        }
> > +    }
> > +
> > +    return 0;
> > +}
> > +
> > +static int blkio_virtio_blk_vhost_vdpa_open(BlockDriverState *bs,
> > +        QDict *options, int flags, Error **errp)
> > +{
> > +    const char *path = qdict_get_try_str(options, "path");
> > +    BDRVBlkioState *s = bs->opaque;
> > +    int ret;
> > +
> > +    ret = blkio_set_str(s->blkio, "path", path);
> > +    qdict_del(options, "path");
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "failed to set path: %s",
> > +                         blkio_get_error_msg());
> > +        return ret;
> > +    }
> > +
> > +    if (flags & BDRV_O_NOCACHE) {
> > +        error_setg(errp, "cache.direct=off is not supported");
> > +        return -EINVAL;
> > +    }
> > +    return 0;
> > +}
> > +
> > +static int blkio_file_open(BlockDriverState *bs, QDict *options, int flags,
> > +                           Error **errp)
> > +{
> > +    const char *blkio_driver = bs->drv->protocol_name;
> > +    BDRVBlkioState *s = bs->opaque;
> > +    int ret;
> > +
> > +    ret = blkio_create(blkio_driver, &s->blkio);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "blkio_create failed: %s",
> > +                         blkio_get_error_msg());
> > +        return ret;
> > +    }
> > +
> > +    if (strcmp(blkio_driver, "io_uring") == 0) {
> > +        ret = blkio_io_uring_open(bs, options, flags, errp);
> > +    } else if (strcmp(blkio_driver, "virtio-blk-vhost-vdpa") == 0) {
> > +        ret = blkio_virtio_blk_vhost_vdpa_open(bs, options, flags, errp);
> > +    }
> > +    if (ret < 0) {
> > +        blkio_destroy(&s->blkio);
> > +        return ret;
> > +    }
> > +
> > +    if (!(flags & BDRV_O_RDWR)) {
> > +        ret = blkio_set_bool(s->blkio, "readonly", true);
> > +        if (ret < 0) {
> > +            error_setg_errno(errp, -ret, "failed to set readonly: %s",
> > +                             blkio_get_error_msg());
> > +            blkio_destroy(&s->blkio);
> > +            return ret;
> > +        }
> > +    }
> > +
> > +    ret = blkio_connect(s->blkio);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "blkio_connect failed: %s",
> > +                         blkio_get_error_msg());
> > +        blkio_destroy(&s->blkio);
> > +        return ret;
> > +    }
> > +
> > +    ret = blkio_get_bool(s->blkio,
> > +                         "needs-mem-regions",
> > +                         &s->needs_mem_regions);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret,
> > +                         "failed to get needs-mem-regions: %s",
> > +                         blkio_get_error_msg());
> > +        blkio_destroy(&s->blkio);
> > +        return ret;
> > +    }
> > +
> > +    ret = blkio_get_uint64(s->blkio,
> > +                           "mem-region-alignment",
> > +                           &s->mem_region_alignment);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret,
> > +                         "failed to get mem-region-alignment: %s",
> > +                         blkio_get_error_msg());
> > +        blkio_destroy(&s->blkio);
> > +        return ret;
> > +    }
> > +
> > +    ret = blkio_start(s->blkio);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "blkio_start failed: %s",
> > +                         blkio_get_error_msg());
> > +        blkio_destroy(&s->blkio);
> > +        return ret;
> > +    }
> > +
> > +    bs->supported_write_flags = BDRV_REQ_FUA;
> > +    bs->supported_zero_flags = BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP |
> > +                               BDRV_REQ_NO_FALLBACK;
> > +
> > +    qemu_mutex_init(&s->lock);
> > +    s->blkioq = blkio_get_queue(s->blkio, 0);
> > +    s->completion_fd = blkioq_get_completion_fd(s->blkioq);
> > +
> > +    blkio_attach_aio_context(bs, bdrv_get_aio_context(bs));
> > +    return 0;
> > +}
> > +
> > +static void blkio_close(BlockDriverState *bs)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +
> > +    qemu_mutex_destroy(&s->lock);
> > +    blkio_destroy(&s->blkio);
> > +}
> > +
> > +static int64_t blkio_getlength(BlockDriverState *bs)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +    uint64_t capacity;
> > +    int ret;
> > +
> > +    WITH_QEMU_LOCK_GUARD(&s->lock) {
> > +        ret = blkio_get_uint64(s->blkio, "capacity", &capacity);
> > +    }
> > +    if (ret < 0) {
> > +        return -ret;
> > +    }
> > +
> > +    return capacity;
> > +}
> > +
> > +static int blkio_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
> > +{
> > +    return 0;
> > +}
> > +
> > +static void blkio_refresh_limits(BlockDriverState *bs, Error **errp)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +    int value;
> > +    int ret;
> > +
> > +    ret = blkio_get_int(s->blkio,
> > +                        "request-alignment",
> > +                        (int *)&bs->bl.request_alignment);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "failed to get \"request-alignment\": %s",
> > +                         blkio_get_error_msg());
> > +        return;
> > +    }
> > +    if (bs->bl.request_alignment < 1 ||
> > +        bs->bl.request_alignment >= INT_MAX ||
> > +        !is_power_of_2(bs->bl.request_alignment)) {
> > +        error_setg(errp, "invalid \"request-alignment\" value %d, must be "
> > +                   "power of 2 less than INT_MAX", bs->bl.request_alignment);
> > +        return;
> > +    }
> > +
> > +    ret = blkio_get_int(s->blkio,
> > +                        "optimal-io-size",
> > +                        (int *)&bs->bl.opt_transfer);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "failed to get \"buf-alignment\": %s",
> > +                         blkio_get_error_msg());
> > +        return;
> > +    }
> > +    if (bs->bl.opt_transfer > INT_MAX ||
> > +        (bs->bl.opt_transfer % bs->bl.request_alignment)) {
> > +        error_setg(errp, "invalid \"buf-alignment\" value %d, must be a "
> > +                   "multiple of %d", bs->bl.opt_transfer,
> > +                   bs->bl.request_alignment);
> > +        return;
> > +    }
> > +
> > +    ret = blkio_get_int(s->blkio,
> > +                        "max-transfer",
> > +                        (int *)&bs->bl.max_transfer);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "failed to get \"max-transfer\": %s",
> > +                         blkio_get_error_msg());
> > +        return;
> > +    }
> > +    if ((bs->bl.max_transfer % bs->bl.request_alignment) ||
> > +        (bs->bl.opt_transfer && (bs->bl.max_transfer % bs->bl.opt_transfer))) {
> > +        error_setg(errp, "invalid \"max-transfer\" value %d, must be a "
> > +                   "multiple of %d and %d (if non-zero)",
> > +                   bs->bl.max_transfer, bs->bl.request_alignment,
> > +                   bs->bl.opt_transfer);
> > +        return;
> > +    }
> > +
> > +    ret = blkio_get_int(s->blkio, "buf-alignment", &value);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "failed to get \"buf-alignment\": %s",
> > +                         blkio_get_error_msg());
> > +        return;
> > +    }
> > +    if (value < 1) {
> > +        error_setg(errp, "invalid \"buf-alignment\" value %d, must be "
> > +                   "positive", value);
> > +        return;
> > +    }
> > +    bs->bl.min_mem_alignment = value;
> > +
> > +    ret = blkio_get_int(s->blkio, "optimal-buf-alignment", &value);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret,
> > +                         "failed to get \"optimal-buf-alignment\": %s",
> > +                         blkio_get_error_msg());
> > +        return;
> > +    }
> > +    if (value < 1) {
> > +        error_setg(errp, "invalid \"optimal-buf-alignment\" value %d, "
> > +                   "must be positive", value);
> > +        return;
> > +    }
> > +    bs->bl.opt_mem_alignment = value;
> > +
> > +    ret = blkio_get_int(s->blkio, "max-segments", &bs->bl.max_iov);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret, "failed to get \"max-segments\": %s",
> > +                         blkio_get_error_msg());
> > +        return;
> > +    }
> > +    if (value < 1) {
> > +        error_setg(errp, "invalid \"max-segments\" value %d, must be positive",
> > +                   bs->bl.max_iov);
> > +        return;
> > +    }
> > +}
> > +
> > +/*
> > + * TODO
> > + * Missing libblkio APIs:
> > + * - write zeroes
> > + * - discard
> > + * - block_status
> > + * - co_invalidate_cache
> > + *
> > + * Out of scope?
> > + * - create
> > + * - truncate
> > + */
> > +
> > +static BlockDriver bdrv_io_uring = {
> > +    .format_name                = "io_uring",
> > +    .protocol_name              = "io_uring",
> > +    .instance_size              = sizeof(BDRVBlkioState),
> > +    .bdrv_needs_filename        = true,
> > +    .bdrv_parse_filename        = blkio_parse_filename_io_uring,
> > +    .bdrv_file_open             = blkio_file_open,
> > +    .bdrv_close                 = blkio_close,
> > +    .bdrv_getlength             = blkio_getlength,
> > +    .bdrv_get_info              = blkio_get_info,
> > +    .bdrv_attach_aio_context    = blkio_attach_aio_context,
> > +    .bdrv_detach_aio_context    = blkio_detach_aio_context,
> > +    .bdrv_aio_pdiscard          = blkio_aio_pdiscard,
> > +    .bdrv_aio_preadv            = blkio_aio_preadv,
> > +    .bdrv_aio_pwritev           = blkio_aio_pwritev,
> > +    .bdrv_aio_flush             = blkio_aio_flush,
> > +    .bdrv_co_pwrite_zeroes      = blkio_co_pwrite_zeroes,
> > +    .bdrv_io_unplug             = blkio_io_unplug,
> > +    .bdrv_refresh_limits        = blkio_refresh_limits,
> > +};
> > +
> > +static BlockDriver bdrv_virtio_blk_vhost_vdpa = {
> > +    .format_name                = "virtio-blk-vhost-vdpa",
> > +    .protocol_name              = "virtio-blk-vhost-vdpa",
> > +    .instance_size              = sizeof(BDRVBlkioState),
> > +    .bdrv_needs_filename        = true,
> 
> Should we set `.bdrv_needs_filename` to false for
> `bdrv_virtio_blk_vhost_vdpa`?
> 
> I have this error:
>     qemu-system-x86_64: -blockdev
> node-name=drive_src1,driver=virtio-blk-vhost-vdpa,path=/dev/vhost-vdpa-0:
> The 'virtio-blk-vhost-vdpa' block driver requires a file name

Yes.

> > +    .bdrv_parse_filename        = blkio_parse_filename_virtio_blk_vhost_vdpa,
> 
> For my education, since virtio-blk-vhost-vdpa doesn't use the filename
> parameter, we still need to set .bdrv_parse_filename?

.bdrv_parse_filename is for converting filename strings to the richer
options structure that QAPI/JSON interfaces offer. We don't need to do
that here and have no "filename" parameter for virtio-blk-vhost-vdpa. I
think it's safe to drop .bdrv_parse_filename().

Stefan

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