[Qemu-devel] [RFC PATCH 12/16] qcow2: Support for feature table header extension

[Kevin Wolf <kwolf@redhat.com>]

Instead of printing an ugly bitmask, qemu can now print a more helpful
string even for yet unknown features.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2.c |   74 ++++++++++++++++++++++++++++++++++++++++++++++++++-------
 block/qcow2.h |   12 +++++++++
 2 files changed, 77 insertions(+), 9 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index aef8282..002e138 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -54,6 +54,7 @@ typedef struct {
 } QCowExtension;
 #define  QCOW2_EXT_MAGIC_END 0
 #define  QCOW2_EXT_MAGIC_BACKING_FORMAT 0xE2792ACA
+#define  QCOW2_EXT_MAGIC_FEATURE_TABLE 0x6803f857
 
 static int qcow2_probe(const uint8_t *buf, int buf_size, const char *filename)
 {
@@ -76,7 +77,7 @@ static int qcow2_probe(const uint8_t *buf, int buf_size, const char *filename)
  * return 0 upon success, non-0 otherwise
  */
 static int qcow2_read_extensions(BlockDriverState *bs, uint64_t start_offset,
-                                 uint64_t end_offset)
+                                 uint64_t end_offset, void **p_feature_table)
 {
     BDRVQcowState *s = bs->opaque;
     QCowExtension ext;
@@ -134,6 +135,18 @@ static int qcow2_read_extensions(BlockDriverState *bs, uint64_t start_offset,
 #endif
             break;
 
+        case QCOW2_EXT_MAGIC_FEATURE_TABLE:
+            if (p_feature_table != NULL) {
+                void* feature_table = g_malloc0(ext.len + 2 * sizeof(Qcow2Feature));
+                ret = bdrv_pread(bs->file, offset , feature_table, ext.len);
+                if (ret < 0) {
+                    return ret;
+                }
+
+                *p_feature_table = feature_table;
+            }
+            break;
+
         default:
             /* unknown magic - save it in case we need to rewrite the header */
             {
@@ -182,6 +195,24 @@ static void report_unsupported(BlockDriverState *bs, const char *fmt, ...)
         bs->device_name, "qcow2", msg);
 }
 
+static void report_unsupported_feature(BlockDriverState *bs,
+    Qcow2Feature *table, uint64_t mask)
+{
+    while (table && table->name[0] != '\0') {
+        if (table->type == QCOW2_FEAT_TYPE_INCOMPATIBLE) {
+            if (mask & (1 << table->bit)) {
+                report_unsupported(bs, "%.46s",table->name);
+                mask &= ~(1 << table->bit);
+            }
+        }
+        table++;
+    }
+
+    if (mask) {
+        report_unsupported(bs, "Unknown incompatible feature: %" PRIx64, mask);
+    }
+}
+
 static int qcow2_open(BlockDriverState *bs, int flags)
 {
     BDRVQcowState *s = bs->opaque;
@@ -245,14 +276,23 @@ static int qcow2_open(BlockDriverState *bs, int flags)
         }
     }
 
+    if (header.backing_file_offset) {
+        ext_end = header.backing_file_offset;
+    } else {
+        ext_end = 1 << header.cluster_bits;
+    }
+
     /* Handle feature bits */
     s->incompatible_features    = header.incompatible_features;
     s->compatible_features      = header.compatible_features;
     s->autoclear_features       = header.autoclear_features;
 
     if (s->incompatible_features != 0) {
-        report_unsupported(bs, "incompatible features mask %" PRIx64,
-                           header.incompatible_features);
+        void *feature_table = NULL;
+        qcow2_read_extensions(bs, header.header_length, ext_end,
+                              &feature_table);
+        report_unsupported_feature(bs, feature_table,
+                                   s->incompatible_features);
         ret = -ENOTSUP;
         goto fail;
     }
@@ -343,12 +383,7 @@ static int qcow2_open(BlockDriverState *bs, int flags)
     QLIST_INIT(&s->cluster_allocs);
 
     /* read qcow2 extensions */
-    if (header.backing_file_offset) {
-        ext_end = header.backing_file_offset;
-    } else {
-        ext_end = s->cluster_size;
-    }
-    if (qcow2_read_extensions(bs, header.header_length, ext_end)) {
+    if (qcow2_read_extensions(bs, header.header_length, ext_end, NULL)) {
         ret = -EINVAL;
         goto fail;
     }
@@ -912,6 +947,27 @@ int qcow2_update_header(BlockDriverState *bs)
         buflen -= ret;
     }
 
+    /* Feature table */
+    Qcow2Feature features[] = {
+        {
+            .type   = QCOW2_FEAT_TYPE_INCOMPATIBLE,
+            .bit    = 0,
+            .name   = "Reference count recovery",
+        }, {
+            .type   = QCOW2_FEAT_TYPE_INCOMPATIBLE,
+            .bit    = 1,
+            .name   = "Subclusters",
+        }
+    };
+
+    ret = header_ext_add(buf, QCOW2_EXT_MAGIC_FEATURE_TABLE,
+                         features, sizeof(features), buflen);
+    if (ret < 0) {
+        goto fail;
+    }
+    buf += ret;
+    buflen -= ret;
+
     /* Keep unknown header extensions */
     QLIST_FOREACH(uext, &s->unknown_header_ext, next) {
         ret = header_ext_add(buf, uext->magic, uext->data, uext->len, buflen);
diff --git a/block/qcow2.h b/block/qcow2.h
index 64cfa69..1adf01d 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -103,6 +103,18 @@ typedef struct Qcow2UnknownHeaderExtension {
     uint8_t data[];
 } Qcow2UnknownHeaderExtension;
 
+enum {
+    QCOW2_FEAT_TYPE_INCOMPATIBLE    = 0,
+    QCOW2_FEAT_TYPE_COMPATIBLE      = 1,
+    QCOW2_FEAT_TYPE_AUTOCLEAR       = 2,
+};
+
+typedef struct Qcow2Feature {
+    uint8_t type;
+    uint8_t bit;
+    char    name[46];
+} QEMU_PACKED Qcow2Feature;
+
 typedef struct BDRVQcowState {
     int cluster_bits;
     int cluster_size;
-- 
1.7.6.5