Given dom0 supports persistent grants but the guest does not.
Then, when attaching a block device during runtime of the guest, dom0
will enable persistent grants for this newly attached block device:

  $ xenstore-ls -f | grep 20674 | grep persistent
  /local/domain/0/backend/vbd/20674/768/feature-persistent = "0"
  /local/domain/0/backend/vbd/20674/51792/feature-persistent = "1"

Here disk 768 was attached during guest creation while 51792 was
attached at runtime. If the guest would have advertised the persistent
grant feature, there would be a xenstore entry like:

  /local/domain/20674/device/vbd/51792/feature-persistent = "1"

Persistent grants are also used when the guest tries to access the disk
which can be seen when enabling log stats:

  $ echo 1 > /sys/module/xen_blkback/parameters/log_stats
  $ dmesg
  xen-blkback: (20674.xvdf-0): oo   0  |  rd    0  |  wr    0  |  f    0 |  ds    0 | pg:    1/1056

The "pg: 1/1056" shows that one persistent grant is used.

Before commit aac8a70db24b ("xen-blkback: add a parameter for disabling
of persistent grants") vbd->feature_gnt_persistent was set in
connect_ring. After the commit it was intended to be initialized in
xen_vbd_create and then set according to the guest feature availability
in connect_ring. However, with a running guest, connect_ring might be
called before xen_vbd_create and vbd->feature_gnt_persistent will be
incorrectly initialized. xen_vbd_create will overwrite it with the value
of feature_persistent regardless whether the guest actually supports
persistent grants.

With this commit, vbd->feature_gnt_persistent is set only in
connect_ring and this is the only use of the module parameter
feature_persistent. This avoids races when the module parameter changes
during the block attachment process.

Note that vbd->feature_gnt_persistent doesn't need to be initialized in
xen_vbd_create. It's next use is in connect which can only be called
once connect_ring has initialized the rings. xen_update_blkif_status is
checking for this.

Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
---
 drivers/block/xen-blkback/xenbus.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 914587aabca0c..51b6ec0380ca4 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -522,8 +522,6 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 	if (q && blk_queue_secure_erase(q))
 		vbd->discard_secure = true;
 
-	vbd->feature_gnt_persistent = feature_persistent;
-
 	pr_debug("Successful creation of handle=%04x (dom=%u)\n",
 		handle, blkif->domid);
 	return 0;
@@ -1090,10 +1088,9 @@ static int connect_ring(struct backend_info *be)
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		return -ENOSYS;
 	}
-	if (blkif->vbd.feature_gnt_persistent)
-		blkif->vbd.feature_gnt_persistent =
-			xenbus_read_unsigned(dev->otherend,
-					"feature-persistent", 0);
+
+	blkif->vbd.feature_gnt_persistent = feature_persistent &&
+		xenbus_read_unsigned(dev->otherend, "feature-persistent", 0);
 
 	blkif->vbd.overflow_max_grants = 0;
 
-- 
2.32.0




Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879

From: SeongJae Park <sjpark@amazon.de>

On Thu, 6 Jan 2022 09:10:13 +0000 Maximilian Heyne <mheyne@amazon.de> wrote:

> Given dom0 supports persistent grants but the guest does not.
> Then, when attaching a block device during runtime of the guest, dom0
> will enable persistent grants for this newly attached block device:
> 
>   $ xenstore-ls -f | grep 20674 | grep persistent
>   /local/domain/0/backend/vbd/20674/768/feature-persistent = "0"
>   /local/domain/0/backend/vbd/20674/51792/feature-persistent = "1"
> 
> Here disk 768 was attached during guest creation while 51792 was
> attached at runtime. If the guest would have advertised the persistent
> grant feature, there would be a xenstore entry like:
> 
>   /local/domain/20674/device/vbd/51792/feature-persistent = "1"
> 
> Persistent grants are also used when the guest tries to access the disk
> which can be seen when enabling log stats:
> 
>   $ echo 1 > /sys/module/xen_blkback/parameters/log_stats
>   $ dmesg
>   xen-blkback: (20674.xvdf-0): oo   0  |  rd    0  |  wr    0  |  f    0 |  ds    0 | pg:    1/1056
> 
> The "pg: 1/1056" shows that one persistent grant is used.
> 
> Before commit aac8a70db24b ("xen-blkback: add a parameter for disabling
> of persistent grants") vbd->feature_gnt_persistent was set in
> connect_ring. After the commit it was intended to be initialized in
> xen_vbd_create and then set according to the guest feature availability
> in connect_ring. However, with a running guest, connect_ring might be
> called before xen_vbd_create and vbd->feature_gnt_persistent will be
> incorrectly initialized. xen_vbd_create will overwrite it with the value
> of feature_persistent regardless whether the guest actually supports
> persistent grants.
> 
> With this commit, vbd->feature_gnt_persistent is set only in
> connect_ring and this is the only use of the module parameter
> feature_persistent. This avoids races when the module parameter changes
> during the block attachment process.
> 
> Note that vbd->feature_gnt_persistent doesn't need to be initialized in
> xen_vbd_create. It's next use is in connect which can only be called
> once connect_ring has initialized the rings. xen_update_blkif_status is
> checking for this.
> 
> Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
> Signed-off-by: Maximilian Heyne <mheyne@amazon.de>

Thank you for this patch!

Reviewed-by: SeongJae Park <sjpark@amazon.de>

Also, I guess this tag is needed?

Cc: <stable@vger.kernel.org> # 5.10.x


Thanks,
SJ

> ---
>  drivers/block/xen-blkback/xenbus.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index 914587aabca0c..51b6ec0380ca4 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -522,8 +522,6 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
>  	if (q && blk_queue_secure_erase(q))
>  		vbd->discard_secure = true;
>  
> -	vbd->feature_gnt_persistent = feature_persistent;
> -
>  	pr_debug("Successful creation of handle=%04x (dom=%u)\n",
>  		handle, blkif->domid);
>  	return 0;
> @@ -1090,10 +1088,9 @@ static int connect_ring(struct backend_info *be)
>  		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
>  		return -ENOSYS;
>  	}
> -	if (blkif->vbd.feature_gnt_persistent)
> -		blkif->vbd.feature_gnt_persistent =
> -			xenbus_read_unsigned(dev->otherend,
> -					"feature-persistent", 0);
> +
> +	blkif->vbd.feature_gnt_persistent = feature_persistent &&
> +		xenbus_read_unsigned(dev->otherend, "feature-persistent", 0);
>  
>  	blkif->vbd.overflow_max_grants = 0;
>  
> -- 
> 2.32.0
> 
> 
> 
> 
> Amazon Development Center Germany GmbH
> Krausenstr. 38
> 10117 Berlin
> Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
> Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
> Sitz: Berlin
> Ust-ID: DE 289 237 879
>