If grant copy is available then it will always be used in preference to
persistent maps. In this case feature-persistent should not be advertized
to the frontend, otherwise it may needlessly copy data into persistently
granted buffers.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
---
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
---
 hw/block/xen_disk.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
index 3a22805fbc..9b06e3aa81 100644
--- a/hw/block/xen_disk.c
+++ b/hw/block/xen_disk.c
@@ -1023,11 +1023,18 @@ static int blk_init(struct XenDevice *xendev)
 
     blkdev->file_blk  = BLOCK_SIZE;
 
+    blkdev->feature_grant_copy =
+                (xengnttab_grant_copy(blkdev->xendev.gnttabdev, 0, NULL) == 0);
+
+    xen_pv_printf(&blkdev->xendev, 3, "grant copy operation %s\n",
+                  blkdev->feature_grant_copy ? "enabled" : "disabled");
+
     /* fill info
      * blk_connect supplies sector-size and sectors
      */
     xenstore_write_be_int(&blkdev->xendev, "feature-flush-cache", 1);
-    xenstore_write_be_int(&blkdev->xendev, "feature-persistent", 1);
+    xenstore_write_be_int(&blkdev->xendev, "feature-persistent",
+                          !blkdev->feature_grant_copy);
     xenstore_write_be_int(&blkdev->xendev, "info", info);
 
     blk_parse_discard(blkdev);
@@ -1202,12 +1209,6 @@ static int blk_connect(struct XenDevice *xendev)
 
     xen_be_bind_evtchn(&blkdev->xendev);
 
-    blkdev->feature_grant_copy =
-                (xengnttab_grant_copy(blkdev->xendev.gnttabdev, 0, NULL) == 0);
-
-    xen_pv_printf(&blkdev->xendev, 3, "grant copy operation %s\n",
-                  blkdev->feature_grant_copy ? "enabled" : "disabled");
-
     xen_pv_printf(&blkdev->xendev, 1, "ok: proto %s, ring-ref %d, "
                   "remote port %d, local port %d\n",
                   blkdev->xendev.protocol, blkdev->ring_ref,
-- 
2.11.0

On Wed, 21 Jun 2017, Paul Durrant wrote:
> If grant copy is available then it will always be used in preference to
> persistent maps. In this case feature-persistent should not be advertized
> to the frontend, otherwise it may needlessly copy data into persistently
> granted buffers.
> 
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> ---
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Max Reitz <mreitz@redhat.com>
> ---
>  hw/block/xen_disk.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
> index 3a22805fbc..9b06e3aa81 100644
> --- a/hw/block/xen_disk.c
> +++ b/hw/block/xen_disk.c
> @@ -1023,11 +1023,18 @@ static int blk_init(struct XenDevice *xendev)
>  
>      blkdev->file_blk  = BLOCK_SIZE;
>  
> +    blkdev->feature_grant_copy =
> +                (xengnttab_grant_copy(blkdev->xendev.gnttabdev, 0, NULL) == 0);
> +
> +    xen_pv_printf(&blkdev->xendev, 3, "grant copy operation %s\n",
> +                  blkdev->feature_grant_copy ? "enabled" : "disabled");
> +
>      /* fill info
>       * blk_connect supplies sector-size and sectors
>       */
>      xenstore_write_be_int(&blkdev->xendev, "feature-flush-cache", 1);
> -    xenstore_write_be_int(&blkdev->xendev, "feature-persistent", 1);
> +    xenstore_write_be_int(&blkdev->xendev, "feature-persistent",
> +                          !blkdev->feature_grant_copy);
>      xenstore_write_be_int(&blkdev->xendev, "info", info);
>  
>      blk_parse_discard(blkdev);
> @@ -1202,12 +1209,6 @@ static int blk_connect(struct XenDevice *xendev)
>  
>      xen_be_bind_evtchn(&blkdev->xendev);
>  
> -    blkdev->feature_grant_copy =
> -                (xengnttab_grant_copy(blkdev->xendev.gnttabdev, 0, NULL) == 0);
> -
> -    xen_pv_printf(&blkdev->xendev, 3, "grant copy operation %s\n",
> -                  blkdev->feature_grant_copy ? "enabled" : "disabled");
> -
>      xen_pv_printf(&blkdev->xendev, 1, "ok: proto %s, ring-ref %d, "
>                    "remote port %d, local port %d\n",
>                    blkdev->xendev.protocol, blkdev->ring_ref,
> -- 
> 2.11.0
>