copy-on-read: add filter drop function

Provide API for the COR-filter removal. Also, drop the filter child
permissions for an inactive state when the filter node is being
removed.
To insert the filter, the block generic layer function
bdrv_insert_node() can be used.
The new function bdrv_cor_filter_drop() may be considered as an
intermediate solution before the QEMU permission update system has
overhauled. Then we are able to implement the API function
bdrv_remove_node() on the block generic layer.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201216061703.70908-4-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/copy-on-read.c

@@ -23,11 +23,20 @@
 #include "qemu/osdep.h"
 #include "block/block_int.h"
 #include "qemu/module.h"
+#include "qapi/error.h"
+#include "block/copy-on-read.h"
+
+
+typedef struct BDRVStateCOR {
+    bool active;
+} BDRVStateCOR;
 
 
 static int cor_open(BlockDriverState *bs, QDict *options, int flags,
                     Error **errp)
 {
+    BDRVStateCOR *state = bs->opaque;
+
     bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
                                BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
                                false, errp);
@@ -42,6 +51,13 @@ static int cor_open(BlockDriverState *bs, QDict *options, int flags,
         ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
             bs->file->bs->supported_zero_flags);
 
+    state->active = true;
+
+    /*
+     * We don't need to call bdrv_child_refresh_perms() now as the permissions
+     * will be updated later when the filter node gets its parent.
+     */
+
     return 0;
 }
 
@@ -57,6 +73,17 @@ static void cor_child_perm(BlockDriverState *bs, BdrvChild *c,
                            uint64_t perm, uint64_t shared,
                            uint64_t *nperm, uint64_t *nshared)
 {
+    BDRVStateCOR *s = bs->opaque;
+
+    if (!s->active) {
+        /*
+         * While the filter is being removed
+         */
+        *nperm = 0;
+        *nshared = BLK_PERM_ALL;
+        return;
+    }
+
     *nperm = perm & PERM_PASSTHROUGH;
     *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
 
@@ -135,6 +162,7 @@ static void cor_lock_medium(BlockDriverState *bs, bool locked)
 
 static BlockDriver bdrv_copy_on_read = {
     .format_name                        = "copy-on-read",
+    .instance_size                      = sizeof(BDRVStateCOR),
 
     .bdrv_open                          = cor_open,
     .bdrv_child_perm                    = cor_child_perm,
@@ -154,6 +182,34 @@ static BlockDriver bdrv_copy_on_read = {
     .is_filter                          = true,
 };
 
+
+void bdrv_cor_filter_drop(BlockDriverState *cor_filter_bs)
+{
+    BdrvChild *child;
+    BlockDriverState *bs;
+    BDRVStateCOR *s = cor_filter_bs->opaque;
+
+    child = bdrv_filter_child(cor_filter_bs);
+    if (!child) {
+        return;
+    }
+    bs = child->bs;
+
+    /* Retain the BDS until we complete the graph change. */
+    bdrv_ref(bs);
+    /* Hold a guest back from writing while permissions are being reset. */
+    bdrv_drained_begin(bs);
+    /* Drop permissions before the graph change. */
+    s->active = false;
+    bdrv_child_refresh_perms(cor_filter_bs, child, &error_abort);
+    bdrv_replace_node(cor_filter_bs, bs, &error_abort);
+
+    bdrv_drained_end(bs);
+    bdrv_unref(bs);
+    bdrv_unref(cor_filter_bs);
+}
+
+
 static void bdrv_copy_on_read_init(void)
 {
     bdrv_register(&bdrv_copy_on_read);

block/copy-on-read.h

@@ -0,0 +1,32 @@
+/*
+ * Copy-on-read filter block driver
+ *
+ * The filter driver performs Copy-On-Read (COR) operations
+ *
+ * Copyright (c) 2018-2020 Virtuozzo International GmbH.
+ *
+ * Author:
+ *   Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef BLOCK_COPY_ON_READ
+#define BLOCK_COPY_ON_READ
+
+#include "block/block_int.h"
+
+void bdrv_cor_filter_drop(BlockDriverState *cor_filter_bs);
+
+#endif /* BLOCK_COPY_ON_READ */