[Qemu-devel] [PATCH v4 3/6] virtio-blk: add "discard" and "write-zeroes" properties

[Stefano Garzarella <sgarzare@redhat.com>]

In order to avoid migration issues, we enable DISCARD and
WRITE_ZEROES features only for machine type >= 4.0

As discussed with Michael S. Tsirkin and Stefan Hajnoczi on the
list [1], DISCARD operation should not have security implications
(eg. page cache attacks), so we can enable it by default.

[1] https://lists.gnu.org/archive/html/qemu-devel/2019-02/msg00504.html

Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 hw/block/virtio-blk.c | 4 ++++
 hw/core/machine.c     | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 938273c63c..6f2e86264d 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -1026,6 +1026,10 @@ static Property virtio_blk_properties[] = {
     DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 128),
     DEFINE_PROP_LINK("iothread", VirtIOBlock, conf.iothread, TYPE_IOTHREAD,
                      IOThread *),
+    DEFINE_PROP_BIT64("discard", VirtIOBlock, host_features,
+                      VIRTIO_BLK_F_DISCARD, true),
+    DEFINE_PROP_BIT64("write-zeroes", VirtIOBlock, host_features,
+                      VIRTIO_BLK_F_WRITE_ZEROES, true),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/hw/core/machine.c b/hw/core/machine.c
index 077fbd182a..766ca5899d 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -33,6 +33,8 @@ GlobalProperty hw_compat_3_1[] = {
     { "usb-kbd", "serial", "42" },
     { "usb-mouse", "serial", "42" },
     { "usb-kbd", "serial", "42" },
+    { "virtio-blk-device", "discard", "false" },
+    { "virtio-blk-device", "write-zeroes", "false" },
 };
 const size_t hw_compat_3_1_len = G_N_ELEMENTS(hw_compat_3_1);
 
-- 
2.20.1