block: add BlockRAMRegistrar

Emulated devices and other BlockBackend users wishing to take advantage
of blk_register_buf() all have the same repetitive job: register
RAMBlocks with the BlockBackend using RAMBlockNotifier.

Add a BlockRAMRegistrar API to do this. A later commit will use this
from hw/block/virtio-blk.c.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20221013185908.1297568-10-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

4 files changed, 97 insertions, 0 deletions

diff --git a/MAINTAINERS b/MAINTAINERS
index 8b4363c0ff..cda146ebbb 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2510,6 +2510,7 @@ F: block*
 F: block/
 F: hw/block/
 F: include/block/
+F: include/sysemu/block-*.h
 F: qemu-img*
 F: docs/tools/qemu-img.rst
 F: qemu-io*
diff --git a/block/block-ram-registrar.c b/block/block-ram-registrar.c
new file mode 100644
index 0000000000..25dbafa789
--- /dev/null
+++ b/block/block-ram-registrar.c
@@ -0,0 +1,58 @@
+/*
+ * BlockBackend RAM Registrar
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "sysemu/block-backend.h"
+#include "sysemu/block-ram-registrar.h"
+#include "qapi/error.h"
+
+static void ram_block_added(RAMBlockNotifier *n, void *host, size_t size,
+                            size_t max_size)
+{
+    BlockRAMRegistrar *r = container_of(n, BlockRAMRegistrar, notifier);
+    Error *err = NULL;
+
+    if (!r->ok) {
+        return; /* don't try again if we've already failed */
+    }
+
+    if (!blk_register_buf(r->blk, host, max_size, &err)) {
+        error_report_err(err);
+        ram_block_notifier_remove(&r->notifier);
+        r->ok = false;
+    }
+}
+
+static void ram_block_removed(RAMBlockNotifier *n, void *host, size_t size,
+                              size_t max_size)
+{
+    BlockRAMRegistrar *r = container_of(n, BlockRAMRegistrar, notifier);
+    blk_unregister_buf(r->blk, host, max_size);
+}
+
+void blk_ram_registrar_init(BlockRAMRegistrar *r, BlockBackend *blk)
+{
+    r->blk = blk;
+    r->notifier = (RAMBlockNotifier){
+        .ram_block_added = ram_block_added,
+        .ram_block_removed = ram_block_removed,
+
+        /*
+         * .ram_block_resized() is not necessary because we use the max_size
+         * value that does not change across resize.
+         */
+    };
+    r->ok = true;
+
+    ram_block_notifier_add(&r->notifier);
+}
+
+void blk_ram_registrar_destroy(BlockRAMRegistrar *r)
+{
+    if (r->ok) {
+        ram_block_notifier_remove(&r->notifier);
+    }
+}
diff --git a/block/meson.build b/block/meson.build
index 500878f082..b7c68b83a3 100644
--- a/block/meson.build
+++ b/block/meson.build
@@ -46,6 +46,7 @@ block_ss.add(files(
 ), zstd, zlib, gnutls)
 
 softmmu_ss.add(when: 'CONFIG_TCG', if_true: files('blkreplay.c'))
+softmmu_ss.add(files('block-ram-registrar.c'))
 
 if get_option('qcow1').allowed()
   block_ss.add(files('qcow.c'))
diff --git a/include/sysemu/block-ram-registrar.h b/include/sysemu/block-ram-registrar.h
new file mode 100644
index 0000000000..d8b2f7942b
--- /dev/null
+++ b/include/sysemu/block-ram-registrar.h
@@ -0,0 +1,37 @@
+/*
+ * BlockBackend RAM Registrar
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#ifndef BLOCK_RAM_REGISTRAR_H
+#define BLOCK_RAM_REGISTRAR_H
+
+#include "exec/ramlist.h"
+
+/**
+ * struct BlockRAMRegistrar:
+ *
+ * Keeps RAMBlock memory registered with a BlockBackend using
+ * blk_register_buf() including hotplugged memory.
+ *
+ * Emulated devices or other BlockBackend users initialize a BlockRAMRegistrar
+ * with blk_ram_registrar_init() before submitting I/O requests with the
+ * BDRV_REQ_REGISTERED_BUF flag set.
+ */
+typedef struct {
+    BlockBackend *blk;
+    RAMBlockNotifier notifier;
+    bool ok;
+} BlockRAMRegistrar;
+
+void blk_ram_registrar_init(BlockRAMRegistrar *r, BlockBackend *blk);
+void blk_ram_registrar_destroy(BlockRAMRegistrar *r);
+
+/* Have all RAMBlocks been registered successfully? */
+static inline bool blk_ram_registrar_ok(BlockRAMRegistrar *r)
+{
+    return r->ok;
+}
+
+#endif /* BLOCK_RAM_REGISTRAR_H */