[GEDI] [PATCH v6 09/13] block: add BlockRAMRegistrar

[Stefano Garzarella]

On Thu, Oct 06, 2022 at 05:35:03PM -0400, Stefan Hajnoczi wrote:
>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 at redhat.com>
>---
> MAINTAINERS                          |  1 +
> include/sysemu/block-ram-registrar.h | 37 ++++++++++++++++++
> block/block-ram-registrar.c          | 58 ++++++++++++++++++++++++++++
> block/meson.build                    |  1 +
> 4 files changed, 97 insertions(+)
> create mode 100644 include/sysemu/block-ram-registrar.h
> create mode 100644 block/block-ram-registrar.c
>
>diff --git a/MAINTAINERS b/MAINTAINERS
>index 0dcae6168a..91aed2cdc7 100644
>--- a/MAINTAINERS
>+++ b/MAINTAINERS
>@@ -2498,6 +2498,7 @@ 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/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 */
>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 */
>+    }

The segfault I was seeing is gone, though, and I'm getting a doubt.

Here we basically just report the error and prevent new regions from 
being registered. The VM still starts though and the blkio driver works 
as if nothing happened.

For drivers that require all regions to be registered, this can cause 
problems, so should we stop the VM in case of failure or put the blkio 
driver in a state such that IOs are not submitted?

Or maybe it's okay and then the device will somehow report the error 
when it can't find the mapped region?

Thanks,
Stefano

>+
>+    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'))
>-- 
>2.37.3
>