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Re: [RFC v2] Adding block layer APIs resembling Linux ZoneBlockDevice io
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From: |
Sam Li |
|
Subject: |
Re: [RFC v2] Adding block layer APIs resembling Linux ZoneBlockDevice ioctls. |
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Date: |
Fri, 24 Jun 2022 11:14:32 +0800 |
Hi Stefan,
Stefan Hajnoczi <stefanha@redhat.com> 于2022年6月20日周一 15:55写道：
>
> On Mon, Jun 20, 2022 at 11:36:11AM +0800, Sam Li wrote:
>
> Hi Sam,
> Is this version 2 of "[RFC v1] Add support for zoned device"? Please
> keep the email subject line the same (except for "v2", "v3", etc) so
> that it's clear which patch series this new version replaces.
>
> > Fix some mistakes before. It can report a range of zones now.
>
> This looks like the description of what changed compared to v1. Please
> put the changelog below "---" in the future. When patch emails are
> merged by git-am(1) it keeps the text above "---" and discards the text
> below "---". The changelog is usually no longer useful once the patches
> are merged, so it should be located below the "---" line.
>
> The text above the "---" is the commit description (an explanation of
> why this commit is necessary). In this case the commit description
> should explain that this patch adds .bdrv_co_zone_report() and
> .bdrv_co_zone_mgmt() to BlockDriver so that zoned block devices can be
> supported.
>
> >
> > Signed-off-by: Sam Li <faithilikerun@gmail.com>
> > ---
> > block/block-backend.c | 22 ++++
> > block/coroutines.h | 5 +
> > block/file-posix.c | 182 ++++++++++++++++++++++++++++++
> > block/io.c | 23 ++++
> > include/block/block-common.h | 43 ++++++-
> > include/block/block-io.h | 13 +++
> > include/block/block_int-common.h | 20 ++++
> > qemu-io-cmds.c | 118 +++++++++++++++++++
> > tests/qemu-iotests/tests/zoned.sh | 52 +++++++++
> > 9 files changed, 477 insertions(+), 1 deletion(-)
> > create mode 100644 tests/qemu-iotests/tests/zoned.sh
> >
> > diff --git a/block/block-backend.c b/block/block-backend.c
> > index e0e1aff4b1..20248e4a35 100644
> > --- a/block/block-backend.c
> > +++ b/block/block-backend.c
> > @@ -104,6 +104,8 @@ typedef struct BlockBackendAIOCB {
> > int ret;
> > } BlockBackendAIOCB;
> >
> > +
> > +
>
> Please avoid whitespace changes in code that is otherwise untouched by
> your patch. Code changes can cause merge conflicts and they make it
> harder to use git-annotate(1), so only changes that are necessary should
> be included in a patch.
>
> > static const AIOCBInfo block_backend_aiocb_info = {
> > .get_aio_context = blk_aiocb_get_aio_context,
> > .aiocb_size = sizeof(BlockBackendAIOCB),
> > @@ -1810,6 +1812,25 @@ int blk_flush(BlockBackend *blk)
> > return ret;
> > }
> >
>
> Please add a documentation comment for blk_co_zone_report() that
> explains how to use the functions and the purpose of the arguments. For
> example, does offset have to be the first byte in a zone or can it be
> any byte offset? What are the alignment requirements of offset and len?
> Why is nr_zones a pointer?
>
> > +int blk_co_zone_report(BlockBackend *blk, int64_t offset, int64_t len,
>
> Functions that run in coroutine context must be labeled with
> coroutine_fn:
>
> int coroutine_fn blk_co_zone_report(...)
>
> This tells humans and tools that the function can only be called from a
> coroutine. There is a blog post about coroutines in QEMU here:
> https://blog.vmsplice.net/2014/01/coroutines-in-qemu-basics.html
>
> > + int64_t *nr_zones,
> > + struct BlockZoneDescriptor *zones)
>
> QEMU coding style uses typedefs when defining structs, so "struct
> BlockZoneDescriptor *zones" should be written as "BlockZoneDescriptor
> *zones".
>
> > +{
> > + int ret;
>
> This function is called from the I/O code path, please mark it with:
>
> IO_CODE();
>
> From include/block/block-io.h:
>
> * I/O API functions. These functions are thread-safe, and therefore
> * can run in any thread as long as the thread has called
> * aio_context_acquire/release().
> *
> * These functions can only call functions from I/O and Common categories,
> * but can be invoked by GS, "I/O or GS" and I/O APIs.
> *
> * All functions in this category must use the macro
> * IO_CODE();
> * to catch when they are accidentally called by the wrong API.
>
> > + ret = bdrv_co_zone_report(blk->root->bs, offset, len, nr_zones, zones);
>
> Please add blk_inc_in_flight(blk) and blk_dec_in_flight(blk) around this
> function call to ensure that zone report requests finish before I/O is
> drained (see bdrv_drained_begin()). This is necessary so that it's
> possible to wait for I/O requests, including zone report, to complete.
>
> Similar to blk_co_do_preadv() we need blk_wait_while_drained(blk),
> blk_check_byte_request(), and bdrv_inc_in_flight(bs) before calling
> bdrv_co_zone_report(). bdrv_dec_in_flight(bs) needs to be called after
> bdrv_co_zone_report() returns.
>
After adding similar structure to blk_co_do_preadv(), zone operation
command will always fail at blk_wait_while_drained(blk) because
blk->inflight <= 0. Would it be ok to just remove
blk_wait_while_drained?
> > + return ret;
> > +}
> > +
> > +int blk_co_zone_mgmt(BlockBackend *blk, enum zone_op op,
> > + int64_t offset, int64_t len)
> > +{
> > + int ret;
> > + ret = bdrv_co_zone_mgmt(blk->root->bs, op, offset, len);
> > +
> > + return ret;
> > +}
>
> The same applies to this function.
>
> > +
> > +
> > void blk_drain(BlockBackend *blk)
> > {
> > BlockDriverState *bs = blk_bs(blk);
> > @@ -2634,3 +2655,4 @@ int blk_make_empty(BlockBackend *blk, Error **errp)
> >
> > return bdrv_make_empty(blk->root, errp);
> > }
> > +
>
> Unrelated whitespace change.
>
> > diff --git a/block/coroutines.h b/block/coroutines.h
> > index 830ecaa733..247326255f 100644
> > --- a/block/coroutines.h
> > +++ b/block/coroutines.h
> > @@ -80,6 +80,11 @@ int coroutine_fn
> > blk_co_do_pdiscard(BlockBackend *blk, int64_t offset, int64_t bytes);
> >
> > int coroutine_fn blk_co_do_flush(BlockBackend *blk);
> > +int coroutine_fn blk_co_zone_report(BlockBackend *blk, int64_t offset,
> > + int64_t len, int64_t *nr_zones,
> > + struct BlockZoneDescriptor *zones);
> > +int coroutine_fn blk_co_zone_mgmt(BlockBackend *blk, enum zone_op op,
> > + int64_t offset, int64_t len);
> >
> >
> > /*
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index 48cd096624..71fd21f8ba 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -178,6 +178,137 @@ typedef struct BDRVRawReopenState {
> > bool check_cache_dropped;
> > } BDRVRawReopenState;
> >
> > +/*
> > + * parse_zone - Fill a zone descriptor
> > + */
> > +static inline void parse_zone(struct BlockZoneDescriptor *zone,
> > + struct blk_zone *blkz) {
> > + zone->start = blkz->start << BDRV_SECTOR_BITS;
> > + zone->length = blkz->len << BDRV_SECTOR_BITS;
> > + zone->cap = blkz->capacity << BDRV_SECTOR_BITS;
> > + zone->wp = blkz->wp << BDRV_SECTOR_BITS;
> > + zone->type = blkz->type;
> > + zone->cond = blkz->type;
>
> Should this be "zone->cond = blkz->cond"?
>
> > +}
> > +
> > +/**
> > + * zone report - Get a zone block device's information in the
> > + * form of an array of zone descriptors.
> > + *
> > + * @param bs: passing zone block device file descriptor
> > + * @param zones: Space to hold zone information on reply
> > + * @param offset: the location in the zone block device
> > + * @return 0 on success, -1 on failure
> > + */
> > +static int raw_co_zone_report(BlockDriverState *bs, int64_t offset,
> > int64_t len,
>
> coroutine_fn
>
> > + int64_t *nr_zones,
> > + struct BlockZoneDescriptor *zones) {
> > + BDRVRawState *s = bs->opaque;
> > + struct blk_zone_report *rep;
> > + struct BlockZoneDescriptor bzd;
> > + struct blk_zone *blkz;
> > + int64_t zone_size_mask, end, rep_size, nrz;
> > + int ret, n = 0, i = 0;
> > +
> > + printf("%s\n", "start to report zone");
>
> This looks like debug output. QEMU has a tracing system that you can
> use. See docs/devel/tracing.rst.
>
> > + nrz = *nr_zones;
> > + rep_size = sizeof(struct blk_zone_report) + nrz * sizeof(struct
> > blk_zone);
> > + rep = (struct blk_zone_report *)malloc(rep_size);
>
> Please use g_autofree and g_new(). QEMU avoids direct use of
> malloc(3)/free(3).
>
> > + if (!rep) {
> > + return -1;
> > + }
> > +
> > + zone_size_mask = zone_start_sector - 1;
> > + /* align up */
> > + end = ((offset + len + zone_size_mask) & (~zone_size_mask))
> > + >> BDRV_SECTOR_BITS;
>
> More readable:
>
> end = DIV_ROUND_UP(offset + len, BDRV_SECTOR_SIZE);
>
> > +
> > + blkz = (struct blk_zone *)(rep + 1);
> > + while (offset < end) {
> > + memset(rep, 0, rep_size);
> > + rep->sector = offset;
> > + rep->nr_zones = nrz;
> > +
> > + ret = ioctl(s->fd, BLKREPORTZONE, rep);
>
> Can this ioctl() block? It seems likely. If yes, then the call needs to
> be made from the thread pool to avoid blocking the current thread. See
> raw_thread_pool_submit().
>
> > + if (ret != 0) {
> > + ret = -errno;
> > + error_report("%d: ioctl BLKREPORTZONE at %ld failed %d",
> > + s->fd, offset, errno);
> > + free(rep);
> > + return ret;
> > + }
> > +
> > + if (!rep->nr_zones) {
> > + break;
> > + }
> > +
> > + for (i = 0; i < rep->nr_zones; i++) {
> > + parse_zone(&bzd, &blkz[i]);
> > + if (zones) {
> > + memcpy(&zones[n], &bzd, sizeof(bzd));
>
> n is never incremented so this always overwrites the first element.
>
> > + }
> > + }
> > +
> > + offset = blkz[i].start + blkz[i].len;
> > + }
> > +
>
> Does this function need to update *nr_zones = n before returning? How does
> the caller know how many zones were returned?
>
> > + return ret;
> > +}
> > +
> > +/**
> > + * zone management operations - Execute an operation on a zone
> > + */
> > +static int raw_co_zone_mgmt(BlockDriverState *bs, enum zone_op op,
> > + int64_t offset, int64_t len) {
> > + BDRVRawState *s = bs->opaque;
> > + struct blk_zone_range range;
> > + const char *ioctl_name;
> > + uint64_t ioctl_op;
>
> ioctl()'s second argument is unsigned long request. Please use unsigned
> long to keep the types consistent.
>
> > + int64_t zone_size_mask, end;
> > + int ret;
> > +
> > + zone_size_mask = zone_start_sector - 1;
> > + /* align up */
> > + end = ((offset + len + zone_size_mask) & (~zone_size_mask))
> > + >> BDRV_SECTOR_BITS;
> > + offset = (offset & (~zone_size_mask)) >> BDRV_SECTOR_BITS; /* align
> > down */
> > +
> > + switch (op) {
> > + case zone_open:
> > + ioctl_name = "BLKOPENZONE";
> > + ioctl_op = BLKOPENZONE;
> > + break;
> > + case zone_close:
> > + ioctl_name = "BLKCLOSEZONE";
> > + ioctl_op = BLKCLOSEZONE;
> > + break;
> > + case zone_finish:
> > + ioctl_name = "BLKFINISHZONE";
> > + ioctl_op = BLKFINISHZONE;
> > + break;
> > + case zone_reset:
> > + ioctl_name = "BLKRESETZONE";
> > + ioctl_op = BLKRESETZONE;
> > + break;
> > + default:
> > + error_report("Invalid zone operation 0x%x", op);
> > + errno = -EINVAL;
> > + return -1;
> > + }
> > +
> > + /* Execute the operation */
> > + range.sector = offset;
> > + range.nr_sectors = end - offset;
> > + ret = ioctl(s->fd, ioctl_op, &range);
> > + if (ret != 0) {
> > + error_report("ioctl %s failed %d",
> > + ioctl_name, errno);
> > + return -1;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > static int fd_open(BlockDriverState *bs)
> > {
> > BDRVRawState *s = bs->opaque;
> > @@ -3324,6 +3455,9 @@ BlockDriver bdrv_file = {
> > .bdrv_abort_perm_update = raw_abort_perm_update,
> > .create_opts = &raw_create_opts,
> > .mutable_opts = mutable_opts,
> > +
> > + .bdrv_co_zone_report = raw_co_zone_report,
> > + .bdrv_co_zone_mgmt = raw_co_zone_mgmt,
> > };
> >
> > /***********************************************/
> > @@ -3703,6 +3837,53 @@ static BlockDriver bdrv_host_device = {
> > #endif
> > };
> >
> > +static BlockDriver bdrv_zoned_host_device = {
> > + .format_name = "zoned_host_device",
> > + .protocol_name = "zoned_host_device",
> > + .instance_size = sizeof(BDRVRawState),
> > + .bdrv_needs_filename = true,
> > + .bdrv_probe_device = hdev_probe_device,
> > + .bdrv_parse_filename = hdev_parse_filename,
> > + .bdrv_file_open = hdev_open,
> > + .bdrv_close = raw_close,
> > + .bdrv_reopen_prepare = raw_reopen_prepare,
> > + .bdrv_reopen_commit = raw_reopen_commit,
> > + .bdrv_reopen_abort = raw_reopen_abort,
> > + .bdrv_co_create_opts = bdrv_co_create_opts_simple,
> > + .create_opts = &bdrv_create_opts_simple,
> > + .mutable_opts = mutable_opts,
> > + .bdrv_co_invalidate_cache = raw_co_invalidate_cache,
> > + .bdrv_co_pwrite_zeroes = hdev_co_pwrite_zeroes,
> > +
> > + .bdrv_co_preadv = raw_co_preadv,
> > + .bdrv_co_pwritev = raw_co_pwritev,
> > + .bdrv_co_flush_to_disk = raw_co_flush_to_disk,
> > + .bdrv_co_pdiscard = hdev_co_pdiscard,
> > + .bdrv_co_copy_range_from = raw_co_copy_range_from,
> > + .bdrv_co_copy_range_to = raw_co_copy_range_to,
> > + .bdrv_refresh_limits = raw_refresh_limits,
> > + .bdrv_io_plug = raw_aio_plug,
> > + .bdrv_io_unplug = raw_aio_unplug,
> > + .bdrv_attach_aio_context = raw_aio_attach_aio_context,
> > +
> > + .bdrv_co_truncate = raw_co_truncate,
> > + .bdrv_getlength = raw_getlength,
> > + .bdrv_get_info = raw_get_info,
> > + .bdrv_get_allocated_file_size
> > + = raw_get_allocated_file_size,
> > + .bdrv_get_specific_stats = hdev_get_specific_stats,
> > + .bdrv_check_perm = raw_check_perm,
> > + .bdrv_set_perm = raw_set_perm,
> > + .bdrv_abort_perm_update = raw_abort_perm_update,
> > + .bdrv_probe_blocksizes = hdev_probe_blocksizes,
> > + .bdrv_probe_geometry = hdev_probe_geometry,
> > + .bdrv_co_ioctl = hdev_co_ioctl,
> > +
> > + /* zone management operations */
> > + .bdrv_co_zone_report = raw_co_zone_report,
> > + .bdrv_co_zone_mgmt = raw_co_zone_mgmt,
> > +};
> > +
> > #if defined(__linux__) || defined(__FreeBSD__) ||
> > defined(__FreeBSD_kernel__)
> > static void cdrom_parse_filename(const char *filename, QDict *options,
> > Error **errp)
> > @@ -3964,6 +4145,7 @@ static void bdrv_file_init(void)
> > #if defined(HAVE_HOST_BLOCK_DEVICE)
> > bdrv_register(&bdrv_host_device);
> > #ifdef __linux__
> > + bdrv_register(&bdrv_zoned_host_device);
> > bdrv_register(&bdrv_host_cdrom);
> > #endif
> > #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
> > diff --git a/block/io.c b/block/io.c
> > index 789e6373d5..3e8bb83cc3 100644
> > --- a/block/io.c
> > +++ b/block/io.c
> > @@ -3258,6 +3258,29 @@ out:
> > return co.ret;
> > }
> >
> > +int bdrv_co_zone_report(BlockDriverState *bs,
> > + int64_t offset, int64_t len, int64_t *nr_zones,
> > + struct BlockZoneDescriptor *zones)
> > +{
> > + int ret = 0;
> > + if (!bs->drv->bdrv_co_zone_report(bs, offset, len, nr_zones, zones)) {
> > + return -ENOTSUP;
> > + }
>
> This returns -ENOTSUP any time ->bdrv_co_zone_report() returns 0. Should
> this be:
>
> if (!bs->drv->bdrv_co_zone_report) {
> return -ENOTSUP;
> }
>
> return bs->drv->bdrv_co_zone_report(bs, offset, len, nr_zones, zones);
>
> ?
>
> > +
> > + return ret;
> > +}
> > +
> > +int bdrv_co_zone_mgmt(BlockDriverState *bs, enum zone_op op,
> > + int64_t offset, int64_t len)
> > +{
> > + int ret = 0;
> > + if (!bs->drv->bdrv_co_zone_mgmt(bs, op, offset, len)) {
> > + return -ENOTSUP;
> > + }
> > +
> > + return ret;
> > +}
> > +
> > void *qemu_blockalign(BlockDriverState *bs, size_t size)
> > {
> > IO_CODE();
> > diff --git a/include/block/block-common.h b/include/block/block-common.h
> > index fdb7306e78..24c468d8ad 100644
> > --- a/include/block/block-common.h
> > +++ b/include/block/block-common.h
> > @@ -23,7 +23,7 @@
> > */
> > #ifndef BLOCK_COMMON_H
> > #define BLOCK_COMMON_H
> > -
> > +#include <linux/blkzoned.h>
>
> Linux-specific code must be #ifdef __linux__ to avoid compilation errors
> on non-Linux hosts.
>
> > #include "block/aio.h"
> > #include "block/aio-wait.h"
> > #include "qemu/iov.h"
> > @@ -48,6 +48,47 @@
> > typedef struct BlockDriver BlockDriver;
> > typedef struct BdrvChild BdrvChild;
> > typedef struct BdrvChildClass BdrvChildClass;
> > +enum zone_type {
>
> Please use "typedef enum { ... } BdrvZoneType" so that enums follow
> QEMU's coding style.
>
> > + BLK_ZT_CONV = BLK_ZONE_TYPE_CONVENTIONAL,
> > + BLK_ZT_SWR = BLK_ZONE_TYPE_SEQWRITE_REQ,
> > + BLK_ZT_SWP = BLK_ZONE_TYPE_SEQWRITE_PREF,
> > +};
> > +
> > +enum zone_cond {
> > + BLK_ZS_NOT_WP = BLK_ZONE_COND_NOT_WP,
> > + BLK_ZS_EMPTY = BLK_ZONE_COND_EMPTY,
> > + BLK_ZS_IOPEN = BLK_ZONE_COND_IMP_OPEN,
> > + BLK_ZS_EOPEN = BLK_ZONE_COND_EXP_OPEN,
> > + BLK_ZS_CLOSED = BLK_ZONE_COND_CLOSED,
> > + BLK_ZS_RDONLY = BLK_ZONE_COND_READONLY,
> > + BLK_ZS_FULL = BLK_ZONE_COND_FULL,
> > + BLK_ZS_OFFLINE = BLK_ZONE_COND_OFFLINE,
> > +};
>
> This 1:1 correspondence with Linux constants could make the code a
> little harder to port.
>
> Maybe QEMU's block layer should define its own numeric constants so the
> code doesn't rely on operating system-specific headers.
> block/file-posix.c #ifdef __linux__ code can be responsible for
> converting Linux-specific constants to QEMU constants (and the 1:1
> mapping can be used there).
>
Can we define those constants in block-common.h? Because
BlockZoneDescriptor requires zone_condition, zone_type defined and
BlockZoneDesicriptor are used in header files and qemu-io
sub-commands. If we use #ifdef __linux__ in block-common.h, it can be
responsible for converting Linux constants instead.
Thanks for reviewing! If there is any problem, please let me know.
Best regards,
Sam
> > +
> > +enum zone_op {
> > + zone_open,
> > + zone_close,
> > + zone_finish,
> > + zone_reset,
> > +};
> > +
> > +/*
> > + * Zone descriptor data structure.
> > + * Provide information on a zone with all position and size values in
> > bytes.
> > + */
> > +typedef struct BlockZoneDescriptor {
> > + uint64_t start;
> > + uint64_t length;
> > + uint64_t cap;
> > + uint64_t wp;
> > + enum zone_type type;
> > + enum zone_cond cond;
> > +} BlockZoneDescriptor;
> > +
> > +enum zone_model {
> > + BLK_Z_HM,
> > + BLK_Z_HA,
> > +};
> >
> > typedef struct BlockDriverInfo {
> > /* in bytes, 0 if irrelevant */
> > diff --git a/include/block/block-io.h b/include/block/block-io.h
> > index 62c84f0519..deb8902684 100644
> > --- a/include/block/block-io.h
> > +++ b/include/block/block-io.h
> > @@ -80,6 +80,13 @@ int bdrv_co_ioctl(BlockDriverState *bs, int req, void
> > *buf);
> > /* Ensure contents are flushed to disk. */
> > int coroutine_fn bdrv_co_flush(BlockDriverState *bs);
> >
> > +/* Report zone information of zone block device. */
> > +int coroutine_fn bdrv_co_zone_report(BlockDriverState *bs, int64_t offset,
> > + int64_t len, int64_t *nr_zones,
> > + struct BlockZoneDescriptor *zones);
> > +int coroutine_fn bdrv_co_zone_mgmt(BlockDriverState *bs, enum zone_op op,
> > + int64_t offset, int64_t len);
> > +
> > int bdrv_co_pdiscard(BdrvChild *child, int64_t offset, int64_t bytes);
> > bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs);
> > int bdrv_block_status(BlockDriverState *bs, int64_t offset,
> > @@ -290,6 +297,12 @@ bdrv_readv_vmstate(BlockDriverState *bs, QEMUIOVector
> > *qiov, int64_t pos);
> > int generated_co_wrapper
> > bdrv_writev_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos);
> >
> > +int generated_co_wrapper blk_zone_report(BlockBackend *blk, int64_t offset,
> > + int64_t len, int64_t *nr_zones,
> > + struct BlockZoneDescriptor
> > *zones);
> > +int generated_co_wrapper blk_zone_mgmt(BlockBackend *blk, enum zone_op op,
> > + int64_t offset, int64_t len);
> > +
> > /**
> > * bdrv_parent_drained_begin_single:
> > *
> > diff --git a/include/block/block_int-common.h
> > b/include/block/block_int-common.h
> > index 8947abab76..4d0180a7da 100644
> > --- a/include/block/block_int-common.h
> > +++ b/include/block/block_int-common.h
> > @@ -63,6 +63,7 @@
> > #define BLOCK_OPT_EXTL2 "extended_l2"
> >
> > #define BLOCK_PROBE_BUF_SIZE 512
> > +#define zone_start_sector 512
> >
> > enum BdrvTrackedRequestType {
> > BDRV_TRACKED_READ,
> > @@ -94,6 +95,19 @@ typedef struct BdrvTrackedRequest {
> > struct BdrvTrackedRequest *waiting_for;
> > } BdrvTrackedRequest;
> >
> > +/**
> > + * Zone device information data structure.
> > + * Provide information on a device.
> > + */
> > +typedef struct zbd_dev {
> > + enum zone_model model;
> > + uint32_t block_size;
>
> How does this relate to QEMU's BlockLimits?
>
> > + uint32_t write_granularity;
>
> Same. Maybe this belongs in BlockLimits?
>
> > + uint32_t nr_zones;
>
> Should this really be limited to 32-bit? For example, take 256 MB zones,
> then the max nr_zones * 256 MB is much smaller than a uint64_t capacity
> value. It seems safer to make this 64-bit, but maybe Dmitry, Hannes, or
> Damien can tell us what to do here.
>
> > + struct BlockZoneDescriptor *zones; /* array of zones */
>
> When is this used?