* [PATCH v2 1/4] block: Improve comments in BlockLimits
2025-10-24 12:30 [PATCH v2 0/4] block: Expose block limits in monitor and qemu-img info Kevin Wolf
@ 2025-10-24 12:30 ` Kevin Wolf
2025-10-24 19:01 ` Eric Blake
2025-10-24 12:30 ` [PATCH v2 2/4] block: Expose block limits for images in QMP Kevin Wolf
` (3 subsequent siblings)
4 siblings, 1 reply; 8+ messages in thread
From: Kevin Wolf @ 2025-10-24 12:30 UTC (permalink / raw)
To: qemu-block; +Cc: kwolf, eblake, armbru, qemu-devel, hreitz
Patches to expose the limits in QAPI have made clear that the existing
documentation of BlockLimits could be improved: The meaning of
min_mem_alignment and opt_mem_alignment could be clearer, and talking
about better alignment values isn't helpful when we only detect these
values and never choose them.
Make the changes in the BlockLimits documentation now, so that the
patches exposing the fields in QAPI can use descriptions consistent with
it.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
include/block/block_int-common.h | 30 +++++++++++++++++-------------
1 file changed, 17 insertions(+), 13 deletions(-)
diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index 034c0634c8..5206f32d53 100644
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -817,10 +817,10 @@ typedef struct BlockLimits {
int64_t max_pdiscard;
/*
- * Optimal alignment for discard requests in bytes. A power of 2
- * is best but not mandatory. Must be a multiple of
- * bl.request_alignment, and must be less than max_pdiscard if
- * that is set. May be 0 if bl.request_alignment is good enough
+ * Optimal alignment for discard requests in bytes. Note that this doesn't
+ * have to be a power of two. Must be a multiple of bl.request_alignment,
+ * and must be less than max_pdiscard if that is set. May be 0 if
+ * bl.request_alignment is good enough.
*/
uint32_t pdiscard_alignment;
@@ -831,11 +831,10 @@ typedef struct BlockLimits {
int64_t max_pwrite_zeroes;
/*
- * Optimal alignment for write zeroes requests in bytes. A power
- * of 2 is best but not mandatory. Must be a multiple of
- * bl.request_alignment, and must be less than max_pwrite_zeroes
- * if that is set. May be 0 if bl.request_alignment is good
- * enough
+ * Optimal alignment for write zeroes requests in bytes. Note that this
+ * doesn't have to be a power of two. Must be a multiple of
+ * bl.request_alignment, and must be less than max_pwrite_zeroes if that is
+ * set. May be 0 if bl.request_alignment is good enough.
*/
uint32_t pwrite_zeroes_alignment;
@@ -863,18 +862,23 @@ typedef struct BlockLimits {
uint64_t max_hw_transfer;
/*
- * Maximal number of scatter/gather elements allowed by the hardware.
+ * Maximum number of scatter/gather elements allowed by the hardware.
* Applies whenever transfers to the device bypass the kernel I/O
* scheduler, for example with SG_IO. If larger than max_iov
* or if zero, blk_get_max_hw_iov will fall back to max_iov.
*/
int max_hw_iov;
-
- /* memory alignment, in bytes so that no bounce buffer is needed */
+ /*
+ * Minimal required memory alignment in bytes for zero-copy I/O to succeed.
+ * For unaligned requests, a bounce buffer will be used.
+ */
size_t min_mem_alignment;
- /* memory alignment, in bytes, for bounce buffer */
+ /*
+ * Optimal memory alignment in bytes. This is the alignment used for any
+ * buffer allocations QEMU performs internally.
+ */
size_t opt_mem_alignment;
/* maximum number of iovec elements */
--
2.51.0
^ permalink raw reply related [flat|nested] 8+ messages in thread* Re: [PATCH v2 1/4] block: Improve comments in BlockLimits
2025-10-24 12:30 ` [PATCH v2 1/4] block: Improve comments in BlockLimits Kevin Wolf
@ 2025-10-24 19:01 ` Eric Blake
0 siblings, 0 replies; 8+ messages in thread
From: Eric Blake @ 2025-10-24 19:01 UTC (permalink / raw)
To: Kevin Wolf; +Cc: qemu-block, armbru, qemu-devel, hreitz
On Fri, Oct 24, 2025 at 02:30:37PM +0200, Kevin Wolf wrote:
> Patches to expose the limits in QAPI have made clear that the existing
> documentation of BlockLimits could be improved: The meaning of
> min_mem_alignment and opt_mem_alignment could be clearer, and talking
> about better alignment values isn't helpful when we only detect these
> values and never choose them.
>
> Make the changes in the BlockLimits documentation now, so that the
> patches exposing the fields in QAPI can use descriptions consistent with
> it.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
> include/block/block_int-common.h | 30 +++++++++++++++++-------------
> 1 file changed, 17 insertions(+), 13 deletions(-)
>
Reviewed-by: Eric Blake <eblake@redhat.com>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization: qemu.org | libguestfs.org
^ permalink raw reply [flat|nested] 8+ messages in thread
* [PATCH v2 2/4] block: Expose block limits for images in QMP
2025-10-24 12:30 [PATCH v2 0/4] block: Expose block limits in monitor and qemu-img info Kevin Wolf
2025-10-24 12:30 ` [PATCH v2 1/4] block: Improve comments in BlockLimits Kevin Wolf
@ 2025-10-24 12:30 ` Kevin Wolf
2025-10-24 12:30 ` [PATCH v2 3/4] qemu-img info: Optionally show block limits Kevin Wolf
` (2 subsequent siblings)
4 siblings, 0 replies; 8+ messages in thread
From: Kevin Wolf @ 2025-10-24 12:30 UTC (permalink / raw)
To: qemu-block; +Cc: kwolf, eblake, armbru, qemu-devel, hreitz
This information can be useful both for debugging and for management
tools trying to configure guest devices with the optimal limits
(possibly across multiple hosts). There is no reason not to make it
available, so just add it to BlockNodeInfo.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
---
qapi/block-core.json | 66 ++++++++++++++++++++++++++++++++
block/qapi.c | 34 ++++++++++++++--
tests/qemu-iotests/184 | 5 ++-
tests/qemu-iotests/184.out | 8 ----
tests/qemu-iotests/common.filter | 3 +-
5 files changed, 102 insertions(+), 14 deletions(-)
diff --git a/qapi/block-core.json b/qapi/block-core.json
index dc6eb4ae23..2c037183f0 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -275,6 +275,69 @@
'file': 'ImageInfoSpecificFileWrapper'
} }
+##
+# @BlockLimitsInfo:
+#
+# @request-alignment: Alignment requirement, in bytes, for
+# offset/length of I/O requests.
+#
+# @max-discard: Maximum number of bytes that can be discarded at once.
+# If not present, there is no specific maximum.
+#
+# @discard-alignment: Optimal alignment for discard requests in bytes.
+# Note that this doesn't have to be a power of two. If not
+# present, discards don't have a alignment requirement different
+# from @request-alignment.
+#
+# @max-write-zeroes: Maximum number of bytes that can be zeroed out at
+# once. If not present, there is no specific maximum.
+#
+# @write-zeroes-alignment: Optimal alignment for write zeroes requests
+# in bytes. Note that this doesn't have to be a power of two. If
+# not present, write_zeroes doesn't have a alignment requirement
+# different from @request-alignment.
+#
+# @opt-transfer: Optimal transfer length in bytes. If not present,
+# there is no preferred size.
+#
+# @max-transfer: Maximal transfer length in bytes. If not present,
+# there is no specific maximum.
+#
+# @max-hw-transfer: Maximal hardware transfer length in bytes.
+# Applies whenever transfers to the device bypass the kernel I/O
+# scheduler, for example with SG_IO. If not present, there is no
+# specific maximum.
+#
+# @max-iov: Maximum number of scatter/gather elements
+#
+# @max-hw-iov: Maximum number of scatter/gather elements allowed by
+# the hardware. Applies whenever transfers to the device bypass
+# the kernel I/O scheduler, for example with SG_IO. If not
+# present, the hardware limits is unknown and @max-iov is always
+# used.
+#
+# @min-mem-alignment: Minimal required memory alignment in bytes for
+# zero-copy I/O to succeed. For unaligned requests, a bounce
+# buffer will be used.
+#
+# @opt-mem-alignment: Optimal memory alignment in bytes. This is the
+# alignment used for any buffer allocations QEMU performs
+# internally.
+##
+{ 'struct': 'BlockLimitsInfo',
+ 'data': { 'request-alignment': 'uint32',
+ '*max-discard': 'uint64',
+ '*discard-alignment': 'uint32',
+ '*max-write-zeroes': 'uint64',
+ '*write-zeroes-alignment': 'uint32',
+ '*opt-transfer': 'uint32',
+ '*max-transfer': 'uint32',
+ '*max-hw-transfer': 'uint32',
+ 'max-iov': 'int',
+ '*max-hw-iov': 'int',
+ 'min-mem-alignment': 'size',
+ 'opt-mem-alignment': 'size' } }
+
##
# @BlockNodeInfo:
#
@@ -304,6 +367,8 @@
#
# @snapshots: list of VM snapshots
#
+# @limits: block limits that are used for I/O on the node (Since 10.2)
+#
# @format-specific: structure supplying additional format-specific
# information (since 1.7)
#
@@ -315,6 +380,7 @@
'*cluster-size': 'int', '*encrypted': 'bool', '*compressed': 'bool',
'*backing-filename': 'str', '*full-backing-filename': 'str',
'*backing-filename-format': 'str', '*snapshots': ['SnapshotInfo'],
+ '*limits': 'BlockLimitsInfo',
'*format-specific': 'ImageInfoSpecific' } }
##
diff --git a/block/qapi.c b/block/qapi.c
index 12fbf8d1b7..54521d0a68 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -235,7 +235,8 @@ int bdrv_query_snapshot_info_list(BlockDriverState *bs,
* in @info, setting @errp on error.
*/
static void GRAPH_RDLOCK
-bdrv_do_query_node_info(BlockDriverState *bs, BlockNodeInfo *info, Error **errp)
+bdrv_do_query_node_info(BlockDriverState *bs, BlockNodeInfo *info, bool limits,
+ Error **errp)
{
int64_t size;
const char *backing_filename;
@@ -269,6 +270,33 @@ bdrv_do_query_node_info(BlockDriverState *bs, BlockNodeInfo *info, Error **errp)
info->dirty_flag = bdi.is_dirty;
info->has_dirty_flag = true;
}
+
+ if (limits) {
+ info->limits = g_new(BlockLimitsInfo, 1);
+ *info->limits = (BlockLimitsInfo) {
+ .request_alignment = bs->bl.request_alignment,
+ .has_max_discard = bs->bl.max_pdiscard != 0,
+ .max_discard = bs->bl.max_pdiscard,
+ .has_discard_alignment = bs->bl.pdiscard_alignment != 0,
+ .discard_alignment = bs->bl.pdiscard_alignment,
+ .has_max_write_zeroes = bs->bl.max_pwrite_zeroes != 0,
+ .max_write_zeroes = bs->bl.max_pwrite_zeroes,
+ .has_write_zeroes_alignment = bs->bl.pwrite_zeroes_alignment != 0,
+ .write_zeroes_alignment = bs->bl.pwrite_zeroes_alignment,
+ .has_opt_transfer = bs->bl.opt_transfer != 0,
+ .opt_transfer = bs->bl.opt_transfer,
+ .has_max_transfer = bs->bl.max_transfer != 0,
+ .max_transfer = bs->bl.max_transfer,
+ .has_max_hw_transfer = bs->bl.max_hw_transfer != 0,
+ .max_hw_transfer = bs->bl.max_hw_transfer,
+ .max_iov = bs->bl.max_iov,
+ .has_max_hw_iov = bs->bl.max_hw_iov != 0,
+ .max_hw_iov = bs->bl.max_hw_iov,
+ .min_mem_alignment = bs->bl.min_mem_alignment,
+ .opt_mem_alignment = bs->bl.opt_mem_alignment,
+ };
+ }
+
info->format_specific = bdrv_get_specific_info(bs, &err);
if (err) {
error_propagate(errp, err);
@@ -343,7 +371,7 @@ void bdrv_query_image_info(BlockDriverState *bs,
ImageInfo *info;
info = g_new0(ImageInfo, 1);
- bdrv_do_query_node_info(bs, qapi_ImageInfo_base(info), errp);
+ bdrv_do_query_node_info(bs, qapi_ImageInfo_base(info), true, errp);
if (*errp) {
goto fail;
}
@@ -397,7 +425,7 @@ void bdrv_query_block_graph_info(BlockDriverState *bs,
BdrvChild *c;
info = g_new0(BlockGraphInfo, 1);
- bdrv_do_query_node_info(bs, qapi_BlockGraphInfo_base(info), errp);
+ bdrv_do_query_node_info(bs, qapi_BlockGraphInfo_base(info), false, errp);
if (*errp) {
goto fail;
}
diff --git a/tests/qemu-iotests/184 b/tests/qemu-iotests/184
index e4cbcd8634..6d0afe9d38 100755
--- a/tests/qemu-iotests/184
+++ b/tests/qemu-iotests/184
@@ -45,8 +45,9 @@ do_run_qemu()
run_qemu()
{
- do_run_qemu "$@" 2>&1 | _filter_testdir | _filter_qemu | _filter_qmp\
- | _filter_qemu_io | _filter_generated_node_ids
+ do_run_qemu "$@" 2>&1 | _filter_testdir | _filter_qemu | _filter_qmp \
+ | _filter_qemu_io | _filter_generated_node_ids \
+ | _filter_img_info
}
test_throttle=$($QEMU_IMG --help|grep throttle)
diff --git a/tests/qemu-iotests/184.out b/tests/qemu-iotests/184.out
index ef99bb2e9a..52692b6b3b 100644
--- a/tests/qemu-iotests/184.out
+++ b/tests/qemu-iotests/184.out
@@ -41,12 +41,6 @@ Testing:
},
"iops_wr": 0,
"ro": false,
- "children": [
- {
- "node-name": "disk0",
- "child": "file"
- }
- ],
"node-name": "throttle0",
"backing_file_depth": 1,
"drv": "throttle",
@@ -75,8 +69,6 @@ Testing:
},
"iops_wr": 0,
"ro": false,
- "children": [
- ],
"node-name": "disk0",
"backing_file_depth": 0,
"drv": "null-co",
diff --git a/tests/qemu-iotests/common.filter b/tests/qemu-iotests/common.filter
index 511a55b1e8..26e6b45b04 100644
--- a/tests/qemu-iotests/common.filter
+++ b/tests/qemu-iotests/common.filter
@@ -229,6 +229,7 @@ _filter_img_info()
discard=0
regex_json_spec_start='^ *"format-specific": \{'
regex_json_child_start='^ *"children": \['
+ regex_json_limit_start='^ *"limits": \{'
gsed -e "s#$REMOTE_TEST_DIR#TEST_DIR#g" \
-e "s#$IMGPROTO:$TEST_DIR#TEST_DIR#g" \
-e "s#$TEST_DIR#TEST_DIR#g" \
@@ -261,7 +262,7 @@ _filter_img_info()
discard=1
elif [[ $line =~ "Child node '/" ]]; then
discard=1
- elif [[ $line =~ $regex_json_spec_start ]]; then
+ elif [[ $line =~ $regex_json_spec_start || $line =~ $regex_json_limit_start ]]; then
discard=2
regex_json_end="^${line%%[^ ]*}\\},? *$"
elif [[ $line =~ $regex_json_child_start ]]; then
--
2.51.0
^ permalink raw reply related [flat|nested] 8+ messages in thread* [PATCH v2 3/4] qemu-img info: Optionally show block limits
2025-10-24 12:30 [PATCH v2 0/4] block: Expose block limits in monitor and qemu-img info Kevin Wolf
2025-10-24 12:30 ` [PATCH v2 1/4] block: Improve comments in BlockLimits Kevin Wolf
2025-10-24 12:30 ` [PATCH v2 2/4] block: Expose block limits for images in QMP Kevin Wolf
@ 2025-10-24 12:30 ` Kevin Wolf
2025-10-24 12:30 ` [PATCH v2 4/4] qemu-img info: Add cache mode option Kevin Wolf
2025-10-24 12:30 ` Kevin Wolf
4 siblings, 0 replies; 8+ messages in thread
From: Kevin Wolf @ 2025-10-24 12:30 UTC (permalink / raw)
To: qemu-block; +Cc: kwolf, eblake, armbru, qemu-devel, hreitz
Add a new --limits option to 'qemu-img info' that displays the block
limits for the image and all of its children, making the information
more accessible for human users than in QMP. This option is not enabled
by default because it can be a lot of output that isn't usually relevant
if you're not specifically trying to diagnose some I/O problem.
This makes the same information automatically also available in HMP
'info block -v'.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
---
docs/tools/qemu-img.rst | 6 +++++-
include/block/qapi.h | 2 +-
block/qapi.c | 34 ++++++++++++++++++++++++++++++++--
qemu-img.c | 15 ++++++++++++---
qemu-img-cmds.hx | 4 ++--
5 files changed, 52 insertions(+), 9 deletions(-)
diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index 5e7b85079d..fdc9ea9cf2 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -503,7 +503,7 @@ Command description:
The size syntax is similar to :manpage:`dd(1)`'s size syntax.
-.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
+.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-U] FILENAME
Give information about the disk image *FILENAME*. Use it in
particular to know the size reserved on disk which can be different
@@ -571,6 +571,10 @@ Command description:
``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2``
for qcow2 images).
+ *Block limits*
+ The block limits for I/O that QEMU detected for the image.
+ This information is only shown if the ``--limits`` option was specified.
+
.. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
Dump the metadata of image *FILENAME* and its backing file chain.
diff --git a/include/block/qapi.h b/include/block/qapi.h
index 54c48de26a..be554e53dc 100644
--- a/include/block/qapi.h
+++ b/include/block/qapi.h
@@ -42,7 +42,7 @@ bdrv_query_image_info(BlockDriverState *bs, ImageInfo **p_info, bool flat,
bool skip_implicit_filters, Error **errp);
void GRAPH_RDLOCK
bdrv_query_block_graph_info(BlockDriverState *bs, BlockGraphInfo **p_info,
- Error **errp);
+ bool limits, Error **errp);
void bdrv_snapshot_dump(QEMUSnapshotInfo *sn);
void bdrv_image_info_specific_dump(ImageInfoSpecific *info_spec,
diff --git a/block/qapi.c b/block/qapi.c
index 54521d0a68..9f5771e019 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -417,6 +417,7 @@ fail:
*/
void bdrv_query_block_graph_info(BlockDriverState *bs,
BlockGraphInfo **p_info,
+ bool limits,
Error **errp)
{
ERRP_GUARD();
@@ -425,7 +426,7 @@ void bdrv_query_block_graph_info(BlockDriverState *bs,
BdrvChild *c;
info = g_new0(BlockGraphInfo, 1);
- bdrv_do_query_node_info(bs, qapi_BlockGraphInfo_base(info), false, errp);
+ bdrv_do_query_node_info(bs, qapi_BlockGraphInfo_base(info), limits, errp);
if (*errp) {
goto fail;
}
@@ -439,7 +440,7 @@ void bdrv_query_block_graph_info(BlockDriverState *bs,
QAPI_LIST_APPEND(children_list_tail, c_info);
c_info->name = g_strdup(c->name);
- bdrv_query_block_graph_info(c->bs, &c_info->info, errp);
+ bdrv_query_block_graph_info(c->bs, &c_info->info, limits, errp);
if (*errp) {
goto fail;
}
@@ -936,6 +937,29 @@ void bdrv_image_info_specific_dump(ImageInfoSpecific *info_spec,
visit_free(v);
}
+/**
+ * Dumps the given BlockLimitsInfo object in a human-readable form,
+ * prepending an optional prefix if the dump is not empty.
+ */
+static void bdrv_image_info_limits_dump(BlockLimitsInfo *limits,
+ const char *prefix,
+ int indentation)
+{
+ QObject *obj;
+ Visitor *v = qobject_output_visitor_new(&obj);
+
+ visit_type_BlockLimitsInfo(v, NULL, &limits, &error_abort);
+ visit_complete(v, &obj);
+ if (!qobject_is_empty_dump(obj)) {
+ if (prefix) {
+ qemu_printf("%*s%s", indentation * 4, "", prefix);
+ }
+ dump_qobject(indentation + 1, obj);
+ }
+ qobject_unref(obj);
+ visit_free(v);
+}
+
/**
* Print the given @info object in human-readable form. Every field is indented
* using the given @indentation (four spaces per indentation level).
@@ -1011,6 +1035,12 @@ void bdrv_node_info_dump(BlockNodeInfo *info, int indentation, bool protocol)
}
}
+ if (info->limits) {
+ bdrv_image_info_limits_dump(info->limits,
+ "Block limits:\n",
+ indentation);
+ }
+
if (info->has_snapshots) {
SnapshotInfoList *elem;
diff --git a/qemu-img.c b/qemu-img.c
index 7a162fdc08..5cdbeda969 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -86,6 +86,7 @@ enum {
OPTION_BITMAPS = 275,
OPTION_FORCE = 276,
OPTION_SKIP_BROKEN = 277,
+ OPTION_LIMITS = 278,
};
typedef enum OutputFormat {
@@ -3002,7 +3003,8 @@ static gboolean str_equal_func(gconstpointer a, gconstpointer b)
static BlockGraphInfoList *collect_image_info_list(bool image_opts,
const char *filename,
const char *fmt,
- bool chain, bool force_share)
+ bool chain, bool limits,
+ bool force_share)
{
BlockGraphInfoList *head = NULL;
BlockGraphInfoList **tail = &head;
@@ -3039,7 +3041,7 @@ static BlockGraphInfoList *collect_image_info_list(bool image_opts,
* the chain manually here.
*/
bdrv_graph_rdlock_main_loop();
- bdrv_query_block_graph_info(bs, &info, &err);
+ bdrv_query_block_graph_info(bs, &info, limits, &err);
bdrv_graph_rdunlock_main_loop();
if (err) {
@@ -3088,6 +3090,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
BlockGraphInfoList *list;
bool image_opts = false;
bool force_share = false;
+ bool limits = false;
fmt = NULL;
for(;;) {
@@ -3097,6 +3100,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
{"image-opts", no_argument, 0, OPTION_IMAGE_OPTS},
{"backing-chain", no_argument, 0, OPTION_BACKING_CHAIN},
{"force-share", no_argument, 0, 'U'},
+ {"limits", no_argument, 0, OPTION_LIMITS},
{"output", required_argument, 0, OPTION_OUTPUT},
{"object", required_argument, 0, OPTION_OBJECT},
{0, 0, 0, 0}
@@ -3119,6 +3123,8 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
" display information about the backing chain for copy-on-write overlays\n"
" -U, --force-share\n"
" open image in shared mode for concurrent access\n"
+" --limits\n"
+" show detected block limits (may depend on options, e.g. cache mode)\n"
" --output human|json\n"
" specify output format (default: human)\n"
" --object OBJDEF\n"
@@ -3140,6 +3146,9 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
case 'U':
force_share = true;
break;
+ case OPTION_LIMITS:
+ limits = true;
+ break;
case OPTION_OUTPUT:
output_format = parse_output_format(argv[0], optarg);
break;
@@ -3156,7 +3165,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
filename = argv[optind++];
list = collect_image_info_list(image_opts, filename, fmt, chain,
- force_share);
+ limits, force_share);
if (!list) {
return 1;
}
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 2c5a8a28f9..74b66f9d42 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -66,9 +66,9 @@ SRST
ERST
DEF("info", img_info,
- "info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [-U] filename")
+ "info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [--limits] [-U] filename")
SRST
-.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
+.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-U] FILENAME
ERST
DEF("map", img_map,
--
2.51.0
^ permalink raw reply related [flat|nested] 8+ messages in thread* [PATCH v2 4/4] qemu-img info: Add cache mode option
2025-10-24 12:30 [PATCH v2 0/4] block: Expose block limits in monitor and qemu-img info Kevin Wolf
` (2 preceding siblings ...)
2025-10-24 12:30 ` [PATCH v2 3/4] qemu-img info: Optionally show block limits Kevin Wolf
@ 2025-10-24 12:30 ` Kevin Wolf
2025-10-24 19:04 ` Eric Blake
2025-10-24 12:30 ` Kevin Wolf
4 siblings, 1 reply; 8+ messages in thread
From: Kevin Wolf @ 2025-10-24 12:30 UTC (permalink / raw)
To: qemu-block; +Cc: kwolf, eblake, armbru, qemu-devel, hreitz
When querying block limits, different cache modes (in particular
O_DIRECT or not) can result in different limits. Add an option to
'qemu-img info' that allows the user to specify a cache mode, so that
they can get the block limits for the cache mode they intend to use with
their VM.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
docs/tools/qemu-img.rst | 2 +-
qemu-img.c | 25 +++++++++++++++++++++----
qemu-img-cmds.hx | 4 ++--
3 files changed, 24 insertions(+), 7 deletions(-)
diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index fdc9ea9cf2..558b0eb84d 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -503,7 +503,7 @@ Command description:
The size syntax is similar to :manpage:`dd(1)`'s size syntax.
-.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-U] FILENAME
+.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-t CACHE] [-U] FILENAME
Give information about the disk image *FILENAME*. Use it in
particular to know the size reserved on disk which can be different
diff --git a/qemu-img.c b/qemu-img.c
index 5cdbeda969..a7791896c1 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3003,6 +3003,7 @@ static gboolean str_equal_func(gconstpointer a, gconstpointer b)
static BlockGraphInfoList *collect_image_info_list(bool image_opts,
const char *filename,
const char *fmt,
+ const char *cache,
bool chain, bool limits,
bool force_share)
{
@@ -3010,6 +3011,15 @@ static BlockGraphInfoList *collect_image_info_list(bool image_opts,
BlockGraphInfoList **tail = &head;
GHashTable *filenames;
Error *err = NULL;
+ int cache_flags = 0;
+ bool writethrough = false;
+ int ret;
+
+ ret = bdrv_parse_cache_mode(cache, &cache_flags, &writethrough);
+ if (ret < 0) {
+ error_report("Invalid cache option: %s", cache);
+ return NULL;
+ }
filenames = g_hash_table_new_full(g_str_hash, str_equal_func, NULL, NULL);
@@ -3026,8 +3036,8 @@ static BlockGraphInfoList *collect_image_info_list(bool image_opts,
g_hash_table_insert(filenames, (gpointer)filename, NULL);
blk = img_open(image_opts, filename, fmt,
- BDRV_O_NO_BACKING | BDRV_O_NO_IO, false, false,
- force_share);
+ BDRV_O_NO_BACKING | BDRV_O_NO_IO | cache_flags,
+ writethrough, false, force_share);
if (!blk) {
goto err;
}
@@ -3087,6 +3097,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
OutputFormat output_format = OFORMAT_HUMAN;
bool chain = false;
const char *filename, *fmt;
+ const char *cache = BDRV_DEFAULT_CACHE;
BlockGraphInfoList *list;
bool image_opts = false;
bool force_share = false;
@@ -3099,13 +3110,14 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
{"format", required_argument, 0, 'f'},
{"image-opts", no_argument, 0, OPTION_IMAGE_OPTS},
{"backing-chain", no_argument, 0, OPTION_BACKING_CHAIN},
+ {"cache", required_argument, 0, 't'},
{"force-share", no_argument, 0, 'U'},
{"limits", no_argument, 0, OPTION_LIMITS},
{"output", required_argument, 0, OPTION_OUTPUT},
{"object", required_argument, 0, OPTION_OBJECT},
{0, 0, 0, 0}
};
- c = getopt_long(argc, argv, "hf:U", long_options, NULL);
+ c = getopt_long(argc, argv, "hf:t:U", long_options, NULL);
if (c == -1) {
break;
}
@@ -3121,6 +3133,8 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
" (incompatible with -f|--format)\n"
" --backing-chain\n"
" display information about the backing chain for copy-on-write overlays\n"
+" -t, --cache CACHE\n"
+" cache mode for FILE (default: " BDRV_DEFAULT_CACHE ")\n"
" -U, --force-share\n"
" open image in shared mode for concurrent access\n"
" --limits\n"
@@ -3143,6 +3157,9 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
case OPTION_BACKING_CHAIN:
chain = true;
break;
+ case 't':
+ cache = optarg;
+ break;
case 'U':
force_share = true;
break;
@@ -3164,7 +3181,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
}
filename = argv[optind++];
- list = collect_image_info_list(image_opts, filename, fmt, chain,
+ list = collect_image_info_list(image_opts, filename, fmt, cache, chain,
limits, force_share);
if (!list) {
return 1;
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 74b66f9d42..6bc8265cfb 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -66,9 +66,9 @@ SRST
ERST
DEF("info", img_info,
- "info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [--limits] [-U] filename")
+ "info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [--limits] [-t CACHE] [-U] filename")
SRST
-.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-U] FILENAME
+.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-t CACHE] [-U] FILENAME
ERST
DEF("map", img_map,
--
2.51.0
^ permalink raw reply related [flat|nested] 8+ messages in thread* Re: [PATCH v2 4/4] qemu-img info: Add cache mode option
2025-10-24 12:30 ` [PATCH v2 4/4] qemu-img info: Add cache mode option Kevin Wolf
@ 2025-10-24 19:04 ` Eric Blake
0 siblings, 0 replies; 8+ messages in thread
From: Eric Blake @ 2025-10-24 19:04 UTC (permalink / raw)
To: Kevin Wolf; +Cc: qemu-block, armbru, qemu-devel, hreitz
On Fri, Oct 24, 2025 at 02:30:40PM +0200, Kevin Wolf wrote:
> When querying block limits, different cache modes (in particular
> O_DIRECT or not) can result in different limits. Add an option to
> 'qemu-img info' that allows the user to specify a cache mode, so that
> they can get the block limits for the cache mode they intend to use with
> their VM.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
> docs/tools/qemu-img.rst | 2 +-
> qemu-img.c | 25 +++++++++++++++++++++----
> qemu-img-cmds.hx | 4 ++--
> 3 files changed, 24 insertions(+), 7 deletions(-)
>
Reviewed-by: Eric Blake <eblake@redhat.com>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization: qemu.org | libguestfs.org
^ permalink raw reply [flat|nested] 8+ messages in thread
* [PATCH v2 4/4] qemu-img info: Add cache mode option
2025-10-24 12:30 [PATCH v2 0/4] block: Expose block limits in monitor and qemu-img info Kevin Wolf
` (3 preceding siblings ...)
2025-10-24 12:30 ` [PATCH v2 4/4] qemu-img info: Add cache mode option Kevin Wolf
@ 2025-10-24 12:30 ` Kevin Wolf
4 siblings, 0 replies; 8+ messages in thread
From: Kevin Wolf @ 2025-10-24 12:30 UTC (permalink / raw)
To: qemu-block; +Cc: kwolf, eblake, armbru, qemu-devel, hreitz
When querying block limits, different cache modes (in particular
O_DIRECT or not) can result in different limits. Add an option to
'qemu-img info' that allows the user to specify a cache mode, so that
they can get the block limits for the cache mode they intend to use with
their VM.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
docs/tools/qemu-img.rst | 2 +-
qemu-img.c | 25 +++++++++++++++++++++----
qemu-img-cmds.hx | 4 ++--
3 files changed, 24 insertions(+), 7 deletions(-)
diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index fdc9ea9cf2..558b0eb84d 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -503,7 +503,7 @@ Command description:
The size syntax is similar to :manpage:`dd(1)`'s size syntax.
-.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-U] FILENAME
+.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-t CACHE] [-U] FILENAME
Give information about the disk image *FILENAME*. Use it in
particular to know the size reserved on disk which can be different
diff --git a/qemu-img.c b/qemu-img.c
index 5cdbeda969..a7791896c1 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3003,6 +3003,7 @@ static gboolean str_equal_func(gconstpointer a, gconstpointer b)
static BlockGraphInfoList *collect_image_info_list(bool image_opts,
const char *filename,
const char *fmt,
+ const char *cache,
bool chain, bool limits,
bool force_share)
{
@@ -3010,6 +3011,15 @@ static BlockGraphInfoList *collect_image_info_list(bool image_opts,
BlockGraphInfoList **tail = &head;
GHashTable *filenames;
Error *err = NULL;
+ int cache_flags = 0;
+ bool writethrough = false;
+ int ret;
+
+ ret = bdrv_parse_cache_mode(cache, &cache_flags, &writethrough);
+ if (ret < 0) {
+ error_report("Invalid cache option: %s", cache);
+ return NULL;
+ }
filenames = g_hash_table_new_full(g_str_hash, str_equal_func, NULL, NULL);
@@ -3026,8 +3036,8 @@ static BlockGraphInfoList *collect_image_info_list(bool image_opts,
g_hash_table_insert(filenames, (gpointer)filename, NULL);
blk = img_open(image_opts, filename, fmt,
- BDRV_O_NO_BACKING | BDRV_O_NO_IO, false, false,
- force_share);
+ BDRV_O_NO_BACKING | BDRV_O_NO_IO | cache_flags,
+ writethrough, false, force_share);
if (!blk) {
goto err;
}
@@ -3087,6 +3097,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
OutputFormat output_format = OFORMAT_HUMAN;
bool chain = false;
const char *filename, *fmt;
+ const char *cache = BDRV_DEFAULT_CACHE;
BlockGraphInfoList *list;
bool image_opts = false;
bool force_share = false;
@@ -3099,13 +3110,14 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
{"format", required_argument, 0, 'f'},
{"image-opts", no_argument, 0, OPTION_IMAGE_OPTS},
{"backing-chain", no_argument, 0, OPTION_BACKING_CHAIN},
+ {"cache", required_argument, 0, 't'},
{"force-share", no_argument, 0, 'U'},
{"limits", no_argument, 0, OPTION_LIMITS},
{"output", required_argument, 0, OPTION_OUTPUT},
{"object", required_argument, 0, OPTION_OBJECT},
{0, 0, 0, 0}
};
- c = getopt_long(argc, argv, "hf:U", long_options, NULL);
+ c = getopt_long(argc, argv, "hf:t:U", long_options, NULL);
if (c == -1) {
break;
}
@@ -3121,6 +3133,8 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
" (incompatible with -f|--format)\n"
" --backing-chain\n"
" display information about the backing chain for copy-on-write overlays\n"
+" -t, --cache CACHE\n"
+" cache mode for FILE (default: " BDRV_DEFAULT_CACHE ")\n"
" -U, --force-share\n"
" open image in shared mode for concurrent access\n"
" --limits\n"
@@ -3143,6 +3157,9 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
case OPTION_BACKING_CHAIN:
chain = true;
break;
+ case 't':
+ cache = optarg;
+ break;
case 'U':
force_share = true;
break;
@@ -3164,7 +3181,7 @@ static int img_info(const img_cmd_t *ccmd, int argc, char **argv)
}
filename = argv[optind++];
- list = collect_image_info_list(image_opts, filename, fmt, chain,
+ list = collect_image_info_list(image_opts, filename, fmt, cache, chain,
limits, force_share);
if (!list) {
return 1;
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 74b66f9d42..6bc8265cfb 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -66,9 +66,9 @@ SRST
ERST
DEF("info", img_info,
- "info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [--limits] [-U] filename")
+ "info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [--limits] [-t CACHE] [-U] filename")
SRST
-.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-U] FILENAME
+.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [--limits] [-t CACHE] [-U] FILENAME
ERST
DEF("map", img_map,
--
2.51.0
^ permalink raw reply related [flat|nested] 8+ messages in thread