[Xen-changelog] [qemu-upstream-unstable] virtio-blk: do not relay a previous driver's WCE configuration to the current

[patchbot@xxxxxxx]

commit c8adc0db7e76e804692372a06ca02cc5a80b67d5
Author:     Paolo Bonzini <pbonzini@xxxxxxxxxx>
AuthorDate: Fri Sep 20 17:31:55 2013 +0200
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue Sep 24 23:03:09 2013 -0500

    virtio-blk: do not relay a previous driver's WCE configuration to the 
current
    
    The following sequence happens:
    - the SeaBIOS virtio-blk driver does not support the WCE feature, which
    causes QEMU to disable writeback caching
    
    - the Linux virtio-blk driver resets the device, finds WCE is available
    but writeback caching is disabled; tells block layer to not send cache
    flush commands
    
    - the Linux virtio-blk driver sets the DRIVER_OK bit, which causes
    writeback caching to be re-enabled, but the Linux virtio-blk driver does
    not know of this side effect and cache flushes remain disabled
    
    The bug is at the third step.  If the guest does know about CONFIG_WCE,
    QEMU should ignore the WCE feature's state.  The guest will control the
    cache mode solely using configuration space.  This change makes Linux
    do flushes correctly, but Linux will keep SeaBIOS's writethrough mode.
    
    Hence, whenever the guest is reset, the cache mode of the disk should
    be reset to whatever was specified in the "-drive" option.  With this
    change, the Linux virtio-blk driver finds that writeback caching is
    enabled, and tells the block layer to send cache flush commands
    appropriately.
    
    Reported-by: Rusty Russell <rusty@xxxxxxxxxxx
    Cc: qemu-stable@xxxxxxxxxx
    Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
    Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
    (cherry picked from commit ef5bc96268ceec64769617dc53b0ac3a20ff351c)
    
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 hw/block/virtio-blk.c          |   24 ++++++++++++++++++++++--
 include/hw/virtio/virtio-blk.h |    1 +
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index e2f55cc..49a23c3 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -460,9 +460,9 @@ static void virtio_blk_dma_restart_cb(void *opaque, int 
running,
 
 static void virtio_blk_reset(VirtIODevice *vdev)
 {
-#ifdef CONFIG_VIRTIO_BLK_DATA_PLANE
     VirtIOBlock *s = VIRTIO_BLK(vdev);
 
+#ifdef CONFIG_VIRTIO_BLK_DATA_PLANE
     if (s->dataplane) {
         virtio_blk_data_plane_stop(s->dataplane);
     }
@@ -473,6 +473,7 @@ static void virtio_blk_reset(VirtIODevice *vdev)
      * are per-device request lists.
      */
     bdrv_drain_all();
+    bdrv_set_enable_write_cache(s->bs, s->original_wce);
 }
 
 /* coalesce internal state, copy to pci i/o region 0
@@ -564,7 +565,25 @@ static void virtio_blk_set_status(VirtIODevice *vdev, 
uint8_t status)
     }
 
     features = vdev->guest_features;
-    bdrv_set_enable_write_cache(s->bs, !!(features & (1 << VIRTIO_BLK_F_WCE)));
+
+    /* A guest that supports VIRTIO_BLK_F_CONFIG_WCE must be able to send
+     * cache flushes.  Thus, the "auto writethrough" behavior is never
+     * necessary for guests that support the VIRTIO_BLK_F_CONFIG_WCE feature.
+     * Leaving it enabled would break the following sequence:
+     *
+     *     Guest started with "-drive cache=writethrough"
+     *     Guest sets status to 0
+     *     Guest sets DRIVER bit in status field
+     *     Guest reads host features (WCE=0, CONFIG_WCE=1)
+     *     Guest writes guest features (WCE=0, CONFIG_WCE=1)
+     *     Guest writes 1 to the WCE configuration field (writeback mode)
+     *     Guest sets DRIVER_OK bit in status field
+     *
+     * s->bs would erroneously be placed in writethrough mode.
+     */
+    if (!(features & (1 << VIRTIO_BLK_F_CONFIG_WCE))) {
+        bdrv_set_enable_write_cache(s->bs, !!(features & (1 << 
VIRTIO_BLK_F_WCE)));
+    }
 }
 
 static void virtio_blk_save(QEMUFile *f, void *opaque)
@@ -674,6 +693,7 @@ static int virtio_blk_device_init(VirtIODevice *vdev)
     }
 
     blkconf_serial(&blk->conf, &blk->serial);
+    s->original_wce = bdrv_enable_write_cache(blk->conf.bs);
     if (blkconf_geometry(&blk->conf, NULL, 65535, 255, 255) < 0) {
         return -1;
     }
diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index b87cf49..41885da 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -123,6 +123,7 @@ typedef struct VirtIOBlock {
     BlockConf *conf;
     VirtIOBlkConf blk;
     unsigned short sector_mask;
+    bool original_wce;
     VMChangeStateEntry *change;
 #ifdef CONFIG_VIRTIO_BLK_DATA_PLANE
     Notifier migration_state_notifier;
--
generated by git-patchbot for /home/xen/git/qemu-upstream-unstable.git

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