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@au1.ibm.com Cc: qemu-stable@nongnu.org Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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@ -460,9 +460,9 @@ static void virtio_blk_dma_restart_cb(void *opaque, int running,
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static void virtio_blk_reset(VirtIODevice *vdev)
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static void virtio_blk_reset(VirtIODevice *vdev)
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{
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{
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#ifdef CONFIG_VIRTIO_BLK_DATA_PLANE
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VirtIOBlock *s = VIRTIO_BLK(vdev);
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VirtIOBlock *s = VIRTIO_BLK(vdev);
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#ifdef CONFIG_VIRTIO_BLK_DATA_PLANE
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if (s->dataplane) {
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if (s->dataplane) {
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virtio_blk_data_plane_stop(s->dataplane);
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virtio_blk_data_plane_stop(s->dataplane);
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}
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}
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@ -473,6 +473,7 @@ static void virtio_blk_reset(VirtIODevice *vdev)
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* are per-device request lists.
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* are per-device request lists.
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*/
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*/
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bdrv_drain_all();
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bdrv_drain_all();
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bdrv_set_enable_write_cache(s->bs, s->original_wce);
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}
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}
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/* coalesce internal state, copy to pci i/o region 0
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/* coalesce internal state, copy to pci i/o region 0
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@ -564,8 +565,26 @@ static void virtio_blk_set_status(VirtIODevice *vdev, uint8_t status)
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}
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}
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features = vdev->guest_features;
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features = vdev->guest_features;
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/* A guest that supports VIRTIO_BLK_F_CONFIG_WCE must be able to send
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* cache flushes. Thus, the "auto writethrough" behavior is never
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* necessary for guests that support the VIRTIO_BLK_F_CONFIG_WCE feature.
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* Leaving it enabled would break the following sequence:
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*
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* Guest started with "-drive cache=writethrough"
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* Guest sets status to 0
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* Guest sets DRIVER bit in status field
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* Guest reads host features (WCE=0, CONFIG_WCE=1)
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* Guest writes guest features (WCE=0, CONFIG_WCE=1)
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* Guest writes 1 to the WCE configuration field (writeback mode)
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* Guest sets DRIVER_OK bit in status field
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*
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* s->bs would erroneously be placed in writethrough mode.
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*/
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if (!(features & (1 << VIRTIO_BLK_F_CONFIG_WCE))) {
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bdrv_set_enable_write_cache(s->bs, !!(features & (1 << VIRTIO_BLK_F_WCE)));
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bdrv_set_enable_write_cache(s->bs, !!(features & (1 << VIRTIO_BLK_F_WCE)));
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}
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}
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}
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static void virtio_blk_save(QEMUFile *f, void *opaque)
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static void virtio_blk_save(QEMUFile *f, void *opaque)
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{
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{
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@ -674,6 +693,7 @@ static int virtio_blk_device_init(VirtIODevice *vdev)
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}
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}
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blkconf_serial(&blk->conf, &blk->serial);
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blkconf_serial(&blk->conf, &blk->serial);
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s->original_wce = bdrv_enable_write_cache(blk->conf.bs);
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if (blkconf_geometry(&blk->conf, NULL, 65535, 255, 255) < 0) {
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if (blkconf_geometry(&blk->conf, NULL, 65535, 255, 255) < 0) {
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return -1;
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return -1;
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}
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}
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@ -123,6 +123,7 @@ typedef struct VirtIOBlock {
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BlockConf *conf;
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BlockConf *conf;
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VirtIOBlkConf blk;
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VirtIOBlkConf blk;
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unsigned short sector_mask;
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unsigned short sector_mask;
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bool original_wce;
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VMChangeStateEntry *change;
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VMChangeStateEntry *change;
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#ifdef CONFIG_VIRTIO_BLK_DATA_PLANE
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#ifdef CONFIG_VIRTIO_BLK_DATA_PLANE
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Notifier migration_state_notifier;
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Notifier migration_state_notifier;
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