{"id":2225924,"url":"http://patchwork.ozlabs.org/api/1.2/patches/2225924/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/patch/20260421202110.306051-5-peterx@redhat.com/","project":{"id":14,"url":"http://patchwork.ozlabs.org/api/1.2/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":"","list_archive_url":"","list_archive_url_format":"","commit_url_format":""},"msgid":"<20260421202110.306051-5-peterx@redhat.com>","list_archive_url":null,"date":"2026-04-21T20:20:58","name":"[v2,04/16] vfio/migration: Cache stop size in VFIOMigration","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"a51d0922f84fcaebd5fae0382030c5cc2befa9d4","submitter":{"id":67717,"url":"http://patchwork.ozlabs.org/api/1.2/people/67717/?format=json","name":"Peter Xu","email":"peterx@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/qemu-devel/patch/20260421202110.306051-5-peterx@redhat.com/mbox/","series":[{"id":500889,"url":"http://patchwork.ozlabs.org/api/1.2/series/500889/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/list/?series=500889","date":"2026-04-21T20:20:55","name":"migration/vfio: Fix a few issues on API misuse or statistic reports","version":2,"mbox":"http://patchwork.ozlabs.org/series/500889/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2225924/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2225924/checks/","tags":{},"related":[],"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=AGZv7/xv;\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=NcjKn/IT;\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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Szmigiero\" ,\n Juraj Marcin ","Subject":"[PATCH v2 04/16] vfio/migration: Cache stop size in VFIOMigration","Date":"Tue, 21 Apr 2026 16:20:58 -0400","Message-ID":"<20260421202110.306051-5-peterx@redhat.com>","X-Mailer":"git-send-email 2.53.0","In-Reply-To":"<20260421202110.306051-1-peterx@redhat.com>","References":"<20260421202110.306051-1-peterx@redhat.com>","MIME-Version":"1.0","Content-Transfer-Encoding":"8bit","Received-SPF":"pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com;\n helo=us-smtp-delivery-124.mimecast.com","X-Spam_score_int":"-20","X-Spam_score":"-2.1","X-Spam_bar":"--","X-Spam_report":"(-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001,\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, SPF_HELO_PASS=-0.001,\n 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":"Add a field to cache stop size. Note that there's an initial value change\nin vfio_save_setup for the stop size default, but it shouldn't matter if it\nis followed with a math of MIN() against VFIO_MIG_DEFAULT_DATA_BUFFER_SIZE.\n\nDocument that all the three sizes we read from VFIO's uAPI on dirty or stop\nsizes are estimates, so QEMU needs to always remember they can be anything.\n\nReviewed-by: Avihai Horon \nSigned-off-by: Peter Xu \n---\n hw/vfio/vfio-migration-internal.h | 8 +++++\n hw/vfio/migration.c | 50 ++++++++++++++++++-------------\n 2 files changed, 38 insertions(+), 20 deletions(-)","diff":"diff --git a/hw/vfio/vfio-migration-internal.h b/hw/vfio/vfio-migration-internal.h\nindex 814fbd9eba..a15fc74703 100644\n--- a/hw/vfio/vfio-migration-internal.h\n+++ b/hw/vfio/vfio-migration-internal.h\n@@ -45,8 +45,16 @@ typedef struct VFIOMigration {\n void *data_buffer;\n size_t data_buffer_size;\n uint64_t mig_flags;\n+ /*\n+ * NOTE: all three sizes cached are reported from VFIO's uAPI, which\n+ * are defined as estimate only. QEMU should not trust these values\n+ * but only use them to do best-effort estimates. Always be prepared\n+ * that these sizes may either grow or even shrink in reality while\n+ * read()ing from the VFIO fds.\n+ */\n uint64_t precopy_init_size;\n uint64_t precopy_dirty_size;\n+ uint64_t stopcopy_size;\n bool multifd_transfer;\n VFIOMultifd *multifd;\n bool initial_data_sent;\ndiff --git a/hw/vfio/migration.c b/hw/vfio/migration.c\nindex 83327b6573..5d5fca09bd 100644\n--- a/hw/vfio/migration.c\n+++ b/hw/vfio/migration.c\n@@ -41,6 +41,12 @@\n */\n #define VFIO_MIG_DEFAULT_DATA_BUFFER_SIZE (1 * MiB)\n \n+/*\n+ * Migration size of VFIO devices can be as little as a few KBs or as big as\n+ * many GBs. This value should be big enough to cover the worst case.\n+ */\n+#define VFIO_MIG_STOP_COPY_SIZE (100 * GiB)\n+\n static unsigned long bytes_transferred;\n \n static const char *mig_state_to_str(enum vfio_device_mig_state state)\n@@ -314,8 +320,7 @@ static void vfio_migration_cleanup(VFIODevice *vbasedev)\n migration->data_fd = -1;\n }\n \n-static int vfio_query_stop_copy_size(VFIODevice *vbasedev,\n- uint64_t *stop_copy_size)\n+static int vfio_query_stop_copy_size(VFIODevice *vbasedev)\n {\n uint64_t buf[DIV_ROUND_UP(sizeof(struct vfio_device_feature) +\n sizeof(struct vfio_device_feature_mig_data_size),\n@@ -323,16 +328,22 @@ static int vfio_query_stop_copy_size(VFIODevice *vbasedev,\n struct vfio_device_feature *feature = (struct vfio_device_feature *)buf;\n struct vfio_device_feature_mig_data_size *mig_data_size =\n (struct vfio_device_feature_mig_data_size *)feature->data;\n+ VFIOMigration *migration = vbasedev->migration;\n \n feature->argsz = sizeof(buf);\n feature->flags =\n VFIO_DEVICE_FEATURE_GET | VFIO_DEVICE_FEATURE_MIG_DATA_SIZE;\n \n if (ioctl(vbasedev->fd, VFIO_DEVICE_FEATURE, feature)) {\n+ /*\n+ * If getting pending migration size fails, VFIO_MIG_STOP_COPY_SIZE\n+ * is reported so downtime limit won't be violated.\n+ */\n+ migration->stopcopy_size = VFIO_MIG_STOP_COPY_SIZE;\n return -errno;\n }\n \n- *stop_copy_size = mig_data_size->stop_copy_length;\n+ migration->stopcopy_size = mig_data_size->stop_copy_length;\n \n return 0;\n }\n@@ -409,6 +420,16 @@ static void vfio_update_estimated_pending_data(VFIOMigration *migration,\n return;\n }\n \n+ /*\n+ * The total size remaining requires separate accounting. Do not trust\n+ * the counter, so what we have read() may be more than what reported.\n+ */\n+ if (migration->stopcopy_size > data_size) {\n+ migration->stopcopy_size -= data_size;\n+ } else {\n+ migration->stopcopy_size = 0;\n+ }\n+\n if (migration->precopy_init_size) {\n uint64_t init_size = MIN(migration->precopy_init_size, data_size);\n \n@@ -463,7 +484,6 @@ static int vfio_save_setup(QEMUFile *f, void *opaque, Error **errp)\n {\n VFIODevice *vbasedev = opaque;\n VFIOMigration *migration = vbasedev->migration;\n- uint64_t stop_copy_size = VFIO_MIG_DEFAULT_DATA_BUFFER_SIZE;\n int ret;\n \n if (!vfio_multifd_setup(vbasedev, false, errp)) {\n@@ -472,9 +492,9 @@ static int vfio_save_setup(QEMUFile *f, void *opaque, Error **errp)\n \n qemu_put_be64(f, VFIO_MIG_FLAG_DEV_SETUP_STATE);\n \n- vfio_query_stop_copy_size(vbasedev, &stop_copy_size);\n+ vfio_query_stop_copy_size(vbasedev);\n migration->data_buffer_size = MIN(VFIO_MIG_DEFAULT_DATA_BUFFER_SIZE,\n- stop_copy_size);\n+ migration->stopcopy_size);\n migration->data_buffer = g_try_malloc0(migration->data_buffer_size);\n if (!migration->data_buffer) {\n error_setg(errp, \"%s: Failed to allocate migration data buffer\",\n@@ -570,32 +590,22 @@ static void vfio_state_pending_estimate(void *opaque, uint64_t *must_precopy,\n migration->precopy_dirty_size);\n }\n \n-/*\n- * Migration size of VFIO devices can be as little as a few KBs or as big as\n- * many GBs. This value should be big enough to cover the worst case.\n- */\n-#define VFIO_MIG_STOP_COPY_SIZE (100 * GiB)\n-\n static void vfio_state_pending_exact(void *opaque, uint64_t *must_precopy,\n uint64_t *can_postcopy)\n {\n VFIODevice *vbasedev = opaque;\n VFIOMigration *migration = vbasedev->migration;\n- uint64_t stop_copy_size = VFIO_MIG_STOP_COPY_SIZE;\n \n- /*\n- * If getting pending migration size fails, VFIO_MIG_STOP_COPY_SIZE is\n- * reported so downtime limit won't be violated.\n- */\n- vfio_query_stop_copy_size(vbasedev, &stop_copy_size);\n- *must_precopy += stop_copy_size;\n+ vfio_query_stop_copy_size(vbasedev);\n+ *must_precopy += migration->stopcopy_size;\n \n if (vfio_device_state_is_precopy(vbasedev)) {\n vfio_query_precopy_size(migration);\n }\n \n trace_vfio_state_pending_exact(vbasedev->name, *must_precopy, *can_postcopy,\n- stop_copy_size, migration->precopy_init_size,\n+ migration->stopcopy_size,\n+ migration->precopy_init_size,\n migration->precopy_dirty_size);\n }\n \n","prefixes":["v2","04/16"]}