{"id":2233212,"url":"http://patchwork.ozlabs.org/api/1.1/patches/2233212/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/patch/20260505202640.1011006-4-peterx@redhat.com/","project":{"id":14,"url":"http://patchwork.ozlabs.org/api/1.1/projects/14/?format=json","name":"QEMU Development","link_name":"qemu-devel","list_id":"qemu-devel.nongnu.org","list_email":"qemu-devel@nongnu.org","web_url":"","scm_url":"","webscm_url":""},"msgid":"<20260505202640.1011006-4-peterx@redhat.com>","date":"2026-05-05T20:26:20","name":"[PULL,03/23] migration/rdma: add x-rdma-chunk-size parameter","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"14e470752ed06eb801bf81eaed2336ab5f674e9d","submitter":{"id":67717,"url":"http://patchwork.ozlabs.org/api/1.1/people/67717/?format=json","name":"Peter Xu","email":"peterx@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/qemu-devel/patch/20260505202640.1011006-4-peterx@redhat.com/mbox/","series":[{"id":502897,"url":"http://patchwork.ozlabs.org/api/1.1/series/502897/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/list/?series=502897","date":"2026-05-05T20:26:17","name":"[PULL,01/23] migration: Fix blocking in POSTCOPY_DEVICE during package load","version":1,"mbox":"http://patchwork.ozlabs.org/series/502897/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2233212/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2233212/checks/","tags":{},"headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=JWTAGjxS;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=google header.b=N81xyfGa;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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envelope-from=peterx@redhat.com;\n helo=us-smtp-delivery-124.mimecast.com","X-Spam_score_int":"-24","X-Spam_score":"-2.5","X-Spam_bar":"--","X-Spam_report":"(-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.443,\n DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1,\n RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001,\n SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no","X-Spam_action":"no action","X-BeenThere":"qemu-devel@nongnu.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"qemu development ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org"},"content":"From: Samuel Zhang \n\nThe default 1MB RDMA chunk size causes slow live migration because\neach chunk triggers a write_flush (ibv_post_send). For 8GB RAM,\n1MB chunk size produces ~15000 flushes vs ~3700 with 1024MB chunk size.\n\nAdd x-rdma-chunk-size parameter to configure the RDMA chunk size for\nfaster migration.\nUsage: `migrate_set_parameter x-rdma-chunk-size 1024M`\n\nPerformance with RDMA live migration of 8GB RAM VM:\n\n| x-rdma-chunk-size (B) | time (s) | throughput (MB/s) |\n|-----------------------|----------|-------------------|\n| 1M (default) | 37.915 | 1,007 |\n| 32M | 17.880 | 2,260 |\n| 1024M | 4.368 | 17,529 |\n\nSigned-off-by: Samuel Zhang \nAcked-by: Markus Armbruster \nAcked-by: Li Zhijian \nTested-by: Li Zhijian \nAcked-by: Fabiano Rosas \nAcked-by: Peter Xu \nLink: https://lore.kernel.org/r/20260427031401.3895523-1-guoqing.zhang@amd.com\nSigned-off-by: Peter Xu \n---\n qapi/migration.json | 13 +++++++++++--\n migration/options.h | 1 +\n migration/migration-hmp-cmds.c | 11 +++++++++++\n migration/options.c | 33 ++++++++++++++++++++++++++++++++-\n migration/rdma.c | 30 ++++++++++++++++--------------\n 5 files changed, 71 insertions(+), 17 deletions(-)","diff":"diff --git a/qapi/migration.json b/qapi/migration.json\nindex 7134d4ce47..0db115ec5e 100644\n--- a/qapi/migration.json\n+++ b/qapi/migration.json\n@@ -806,7 +806,7 @@\n #\n # Features:\n #\n-# @unstable: Members @x-checkpoint-delay and\n+# @unstable: Members @x-checkpoint-delay, @x-rdma-chunk-size, and\n # @x-vcpu-dirty-limit-period are experimental.\n #\n # Since: 2.4\n@@ -831,6 +831,7 @@\n 'mode',\n 'zero-page-detection',\n 'direct-io',\n+ { 'name': 'x-rdma-chunk-size', 'features': [ 'unstable' ] },\n 'cpr-exec-command'] }\n \n ##\n@@ -1007,9 +1008,15 @@\n # is @cpr-exec. The first list element is the program's filename,\n # the remainder its arguments. (Since 10.2)\n #\n+# @x-rdma-chunk-size: RDMA memory registration chunk size in bytes.\n+# Default is 1MiB. Must be a power of 2 in the range\n+# [1MiB, 1024MiB]. Only applies when migrating via RDMA.\n+# Must be set to the same value on both source and destination\n+# before migration starts. (Since 11.1)\n+#\n # Features:\n #\n-# @unstable: Members @x-checkpoint-delay and\n+# @unstable: Members @x-checkpoint-delay, @x-rdma-chunk-size, and\n # @x-vcpu-dirty-limit-period are experimental.\n #\n # Since: 2.4\n@@ -1046,6 +1053,8 @@\n '*mode': 'MigMode',\n '*zero-page-detection': 'ZeroPageDetection',\n '*direct-io': 'bool',\n+ '*x-rdma-chunk-size': { 'type': 'uint64',\n+ 'features': [ 'unstable' ] },\n '*cpr-exec-command': [ 'str' ]} }\n \n ##\ndiff --git a/migration/options.h b/migration/options.h\nindex b502871097..b46221998a 100644\n--- a/migration/options.h\n+++ b/migration/options.h\n@@ -87,6 +87,7 @@ const char *migrate_tls_creds(void);\n const char *migrate_tls_hostname(void);\n uint64_t migrate_xbzrle_cache_size(void);\n ZeroPageDetection migrate_zero_page_detection(void);\n+uint64_t migrate_rdma_chunk_size(void);\n \n /* parameters helpers */\n \ndiff --git a/migration/migration-hmp-cmds.c b/migration/migration-hmp-cmds.c\nindex 0a193b8f54..4f6c1dbf89 100644\n--- a/migration/migration-hmp-cmds.c\n+++ b/migration/migration-hmp-cmds.c\n@@ -451,6 +451,13 @@ void hmp_info_migrate_parameters(Monitor *mon, const QDict *qdict)\n params->direct_io ? \"on\" : \"off\");\n }\n \n+ if (params->has_x_rdma_chunk_size) {\n+ monitor_printf(mon, \"%s: %\" PRIu64 \" bytes\\n\",\n+ MigrationParameter_str(\n+ MIGRATION_PARAMETER_X_RDMA_CHUNK_SIZE),\n+ params->x_rdma_chunk_size);\n+ }\n+\n assert(params->has_cpr_exec_command);\n monitor_print_cpr_exec_command(mon, params->cpr_exec_command);\n }\n@@ -734,6 +741,10 @@ void hmp_migrate_set_parameter(Monitor *mon, const QDict *qdict)\n p->has_direct_io = true;\n visit_type_bool(v, param, &p->direct_io, &err);\n break;\n+ case MIGRATION_PARAMETER_X_RDMA_CHUNK_SIZE:\n+ p->has_x_rdma_chunk_size = true;\n+ visit_type_size(v, param, &p->x_rdma_chunk_size, &err);\n+ break;\n case MIGRATION_PARAMETER_CPR_EXEC_COMMAND: {\n /*\n * NOTE: g_autofree will only auto g_free() the strv array when\ndiff --git a/migration/options.c b/migration/options.c\nindex 68441f0276..5cbfd29099 100644\n--- a/migration/options.c\n+++ b/migration/options.c\n@@ -13,6 +13,7 @@\n \n #include \"qemu/osdep.h\"\n #include \"qemu/error-report.h\"\n+#include \"qemu/units.h\"\n #include \"exec/target_page.h\"\n #include \"qapi/clone-visitor.h\"\n #include \"qapi/error.h\"\n@@ -90,6 +91,7 @@ const PropertyInfo qdev_prop_StrOrNull;\n \n #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT_PERIOD 1000 /* milliseconds */\n #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT 1 /* MB/s */\n+#define DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE MiB\n \n const Property migration_properties[] = {\n DEFINE_PROP_BOOL(\"store-global-state\", MigrationState,\n@@ -183,6 +185,9 @@ const Property migration_properties[] = {\n DEFINE_PROP_ZERO_PAGE_DETECTION(\"zero-page-detection\", MigrationState,\n parameters.zero_page_detection,\n ZERO_PAGE_DETECTION_MULTIFD),\n+ DEFINE_PROP_UINT64(\"x-rdma-chunk-size\", MigrationState,\n+ parameters.x_rdma_chunk_size,\n+ DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE),\n \n /* Migration capabilities */\n DEFINE_PROP_MIG_CAP(\"x-xbzrle\", MIGRATION_CAPABILITY_XBZRLE),\n@@ -1000,6 +1005,15 @@ ZeroPageDetection migrate_zero_page_detection(void)\n return s->parameters.zero_page_detection;\n }\n \n+uint64_t migrate_rdma_chunk_size(void)\n+{\n+ MigrationState *s = migrate_get_current();\n+ uint64_t size = s->parameters.x_rdma_chunk_size;\n+\n+ assert(MiB <= size && size <= GiB && is_power_of_2(size));\n+ return size;\n+}\n+\n /* parameters helpers */\n \n AnnounceParameters *migrate_announce_params(void)\n@@ -1062,7 +1076,7 @@ static void migrate_mark_all_params_present(MigrationParameters *p)\n &p->has_announce_step, &p->has_block_bitmap_mapping,\n &p->has_x_vcpu_dirty_limit_period, &p->has_vcpu_dirty_limit,\n &p->has_mode, &p->has_zero_page_detection, &p->has_direct_io,\n- &p->has_cpr_exec_command,\n+ &p->has_x_rdma_chunk_size, &p->has_cpr_exec_command,\n };\n \n len = ARRAY_SIZE(has_fields);\n@@ -1273,6 +1287,15 @@ bool migrate_params_check(MigrationParameters *params, Error **errp)\n return false;\n }\n \n+ if (params->has_x_rdma_chunk_size &&\n+ (params->x_rdma_chunk_size < MiB ||\n+ params->x_rdma_chunk_size > GiB ||\n+ !is_power_of_2(params->x_rdma_chunk_size))) {\n+ error_setg(errp, \"Option x_rdma_chunk_size expects \"\n+ \"a power of 2 in the range 1MiB to 1024MiB\");\n+ return false;\n+ }\n+\n return true;\n }\n \n@@ -1393,6 +1416,10 @@ static void migrate_params_test_apply(MigrationParameters *params,\n dest->direct_io = params->direct_io;\n }\n \n+ if (params->has_x_rdma_chunk_size) {\n+ dest->x_rdma_chunk_size = params->x_rdma_chunk_size;\n+ }\n+\n if (params->has_cpr_exec_command) {\n qapi_free_strList(dest->cpr_exec_command);\n dest->cpr_exec_command = QAPI_CLONE(strList, params->cpr_exec_command);\n@@ -1520,6 +1547,10 @@ static void migrate_params_apply(MigrationParameters *params)\n s->parameters.direct_io = params->direct_io;\n }\n \n+ if (params->has_x_rdma_chunk_size) {\n+ s->parameters.x_rdma_chunk_size = params->x_rdma_chunk_size;\n+ }\n+\n if (params->has_cpr_exec_command) {\n qapi_free_strList(s->parameters.cpr_exec_command);\n s->parameters.cpr_exec_command =\ndiff --git a/migration/rdma.c b/migration/rdma.c\nindex 55ab85650a..3e37a1d440 100644\n--- a/migration/rdma.c\n+++ b/migration/rdma.c\n@@ -45,10 +45,12 @@\n #define RDMA_RESOLVE_TIMEOUT_MS 10000\n \n /* Do not merge data if larger than this. */\n-#define RDMA_MERGE_MAX (2 * 1024 * 1024)\n-#define RDMA_SIGNALED_SEND_MAX (RDMA_MERGE_MAX / 4096)\n+static inline uint64_t rdma_merge_max(void)\n+{\n+ return migrate_rdma_chunk_size() * 2;\n+}\n \n-#define RDMA_REG_CHUNK_SHIFT 20 /* 1 MB */\n+#define RDMA_SIGNALED_SEND_MAX 512\n \n /*\n * This is only for non-live state being migrated.\n@@ -527,21 +529,21 @@ static int qemu_rdma_exchange_send(RDMAContext *rdma, RDMAControlHeader *head,\n static inline uint64_t ram_chunk_index(const uint8_t *start,\n const uint8_t *host)\n {\n- return ((uintptr_t) host - (uintptr_t) start) >> RDMA_REG_CHUNK_SHIFT;\n+ return ((uintptr_t) host - (uintptr_t) start) / migrate_rdma_chunk_size();\n }\n \n static inline uint8_t *ram_chunk_start(const RDMALocalBlock *rdma_ram_block,\n uint64_t i)\n {\n return (uint8_t *)(uintptr_t)(rdma_ram_block->local_host_addr +\n- (i << RDMA_REG_CHUNK_SHIFT));\n+ (i * migrate_rdma_chunk_size()));\n }\n \n static inline uint8_t *ram_chunk_end(const RDMALocalBlock *rdma_ram_block,\n uint64_t i)\n {\n uint8_t *result = ram_chunk_start(rdma_ram_block, i) +\n- (1UL << RDMA_REG_CHUNK_SHIFT);\n+ migrate_rdma_chunk_size();\n \n if (result > (rdma_ram_block->local_host_addr + rdma_ram_block->length)) {\n result = rdma_ram_block->local_host_addr + rdma_ram_block->length;\n@@ -1841,6 +1843,7 @@ static int qemu_rdma_write_one(RDMAContext *rdma,\n struct ibv_send_wr *bad_wr;\n int reg_result_idx, ret, count = 0;\n uint64_t chunk, chunks;\n+ uint64_t chunk_size = migrate_rdma_chunk_size();\n uint8_t *chunk_start, *chunk_end;\n RDMALocalBlock *block = &(rdma->local_ram_blocks.block[current_index]);\n RDMARegister reg;\n@@ -1861,22 +1864,21 @@ retry:\n chunk_start = ram_chunk_start(block, chunk);\n \n if (block->is_ram_block) {\n- chunks = length / (1UL << RDMA_REG_CHUNK_SHIFT);\n+ chunks = length / chunk_size;\n \n- if (chunks && ((length % (1UL << RDMA_REG_CHUNK_SHIFT)) == 0)) {\n+ if (chunks && ((length % chunk_size) == 0)) {\n chunks--;\n }\n } else {\n- chunks = block->length / (1UL << RDMA_REG_CHUNK_SHIFT);\n+ chunks = block->length / chunk_size;\n \n- if (chunks && ((block->length % (1UL << RDMA_REG_CHUNK_SHIFT)) == 0)) {\n+ if (chunks && ((block->length % chunk_size) == 0)) {\n chunks--;\n }\n }\n \n trace_qemu_rdma_write_one_top(chunks + 1,\n- (chunks + 1) *\n- (1UL << RDMA_REG_CHUNK_SHIFT) / 1024 / 1024);\n+ (chunks + 1) * chunk_size / 1024 / 1024);\n \n chunk_end = ram_chunk_end(block, chunk + chunks);\n \n@@ -2176,7 +2178,7 @@ static int qemu_rdma_write(RDMAContext *rdma,\n rdma->current_length += len;\n \n /* flush it if buffer is too large */\n- if (rdma->current_length >= RDMA_MERGE_MAX) {\n+ if (rdma->current_length >= rdma_merge_max()) {\n return qemu_rdma_write_flush(rdma, errp);\n }\n \n@@ -3522,7 +3524,7 @@ int rdma_registration_handle(QEMUFile *f)\n } else {\n chunk = reg->key.chunk;\n host_addr = block->local_host_addr +\n- (reg->key.chunk * (1UL << RDMA_REG_CHUNK_SHIFT));\n+ (reg->key.chunk * migrate_rdma_chunk_size());\n /* Check for particularly bad chunk value */\n if (host_addr < (void *)block->local_host_addr) {\n error_report(\"rdma: bad chunk for block %s\"\n","prefixes":["PULL","03/23"]}