[Qemu-devel] [PATCH 3/3] disk: don't read from disk until the guest starts

[Anthony Liguori]

This fixes a couple nasty problems relating to live migration.

1) When dealing with shared storage with weak coherence (i.e. NFS), even if
   we re-read, we may end up with undesired caching.  By delaying any reads
   until we absolutely have to, we decrease the likelihood of any undesirable
   caching.

2) When dealing with copy-on-read, the local storage acts as a cache.  We need
   to make sure to avoid any reads to avoid polluting the local cache.

Signed-off-by: Anthony Liguori <address@hidden>

diff --git a/hw/ide/core.c b/hw/ide/core.c
index 1e466d1..57d8db3 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -94,6 +94,33 @@ static void put_le16(uint16_t *p, unsigned int v)
     *p = cpu_to_le16(v);
 }
 
+static int guess_geometry(IDEState *s)
+{
+    int cylinders, heads, secs;
+
+    if (s->cylinders != 0) {
+        return -1;
+    }
+
+    bdrv_guess_geometry(s->bs, &cylinders, &heads, &secs);
+    if (cylinders < 1 || cylinders > 16383) {
+        error_report("cyls must be between 1 and 16383");
+        return -1;
+    }
+    if (heads < 1 || heads > 16) {
+        error_report("heads must be between 1 and 16");
+        return -1;
+    }
+    if (secs < 1 || secs > 63) {
+        error_report("secs must be between 1 and 63");
+        return -1;
+    }
+    s->cylinders = cylinders;
+    s->heads = heads;
+    s->sectors = secs;
+    return 0;
+}
+
 static void ide_identify(IDEState *s)
 {
     uint16_t *p;
@@ -105,6 +132,8 @@ static void ide_identify(IDEState *s)
        return;
     }
 
+    guess_geometry(s);
+
     memset(s->io_buffer, 0, 512);
     p = (uint16_t *)s->io_buffer;
     put_le16(p + 0, 0x0040);
@@ -2614,27 +2643,13 @@ void ide_bus_reset(IDEBus *bus)
 int ide_init_drive(IDEState *s, BlockDriverState *bs,
                    const char *version, const char *serial)
 {
-    int cylinders, heads, secs;
     uint64_t nb_sectors;
 
     s->bs = bs;
-    bdrv_get_geometry(bs, &nb_sectors);
-    bdrv_guess_geometry(bs, &cylinders, &heads, &secs);
-    if (cylinders < 1 || cylinders > 16383) {
-        error_report("cyls must be between 1 and 16383");
-        return -1;
-    }
-    if (heads < 1 || heads > 16) {
-        error_report("heads must be between 1 and 16");
-        return -1;
-    }
-    if (secs < 1 || secs > 63) {
-        error_report("secs must be between 1 and 63");
-        return -1;
-    }
-    s->cylinders = cylinders;
-    s->heads = heads;
-    s->sectors = secs;
+    bdrv_get_geometry(s->bs, &nb_sectors);
+    s->cylinders = 0;
+    s->heads = 0;
+    s->sectors = 0;
     s->nb_sectors = nb_sectors;
     /* The SMART values should be preserved across power cycles
        but they aren't.  */
diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index bd6bbe6..0bf17ec 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -427,6 +427,10 @@ static void virtio_blk_update_config(VirtIODevice *vdev, 
uint8_t *config)
 
     bdrv_get_geometry(s->bs, &capacity);
     bdrv_get_geometry_hint(s->bs, &cylinders, &heads, &secs);
+    if (cylinders == 0) {
+        bdrv_guess_geometry(s->bs, &cylinders, &heads, &secs);
+    }
+
     memset(&blkcfg, 0, sizeof(blkcfg));
     stq_raw(&blkcfg.capacity, capacity);
     stl_raw(&blkcfg.seg_max, 128 - 2);
@@ -501,7 +505,6 @@ static int virtio_blk_load(QEMUFile *f, void *opaque, int 
version_id)
 VirtIODevice *virtio_blk_init(DeviceState *dev, BlockConf *conf)
 {
     VirtIOBlock *s;
-    int cylinders, heads, secs;
     static int virtio_blk_id;
     DriveInfo *dinfo;
 
@@ -525,7 +528,6 @@ VirtIODevice *virtio_blk_init(DeviceState *dev, BlockConf 
*conf)
     s->conf = conf;
     s->rq = NULL;
     s->sector_mask = (s->conf->logical_block_size / BDRV_SECTOR_SIZE) - 1;
-    bdrv_guess_geometry(s->bs, &cylinders, &heads, &secs);
 
     /* NB: per existing s/n string convention the string is terminated
      * by '\0' only when less than sizeof (s->sn)
-- 
1.7.0.4