Re: [PATCH v2] xen-blkback: fix persistent grants negotiation

[Andrii Chepurnyi <andrii.chepurnyi82@xxxxxxxxx>]

Hello All,

I faced the mentioned issue recently and just to bring more context here is our setup:
We use pvblock backend for Android guest. It starts using u-boot with pvblock support(which frontend doesn't support the persistent grants feature), later it loads and starts the Linux kernel(which frontend supports the persistent grants feature). So in total, we have sequent two different frontends reconnection, the first of which doesn't support persistent grants.
So the original patch [1] perfectly solves the original issue and provides the ability to use persistent grants after the reconnection when Linux frontend which supports persistent grants comes into play.
At the same time [2] will disable the persistent grants feature for the first and second frontend.
Is it possible to keep [1]  as is?

[1] https://lore.kernel.org/xen-devel/20220106091013.126076-1-mheyne@xxxxxxxxx/
[2] https://lore.kernel.org/xen-devel/20220714224410.51147-1-sj@xxxxxxxxxx/

Best regards,

Andrii

On Fri, Jul 15, 2022 at 1:15 PM Oleksandr <olekstysh@xxxxxxxxx> wrote:

On 15.07.22 01:44, SeongJae Park wrote:


Hello all.

Adding Andrii Chepurnyi to CC who have played with the use-case which
required reconnect recently and faced some issues with
feature_persistent handling.




> Persistent grants feature can be used only when both backend and the
> frontend supports the feature.  The feature was always supported by
> 'blkback', but commit aac8a70db24b ("xen-blkback: add a parameter for
> disabling of persistent grants") has introduced a parameter for
> disabling it runtime.
>
> To avoid the parameter be updated while being used by 'blkback', the
> commit caches the parameter into 'vbd->feature_gnt_persistent' in
> 'xen_vbd_create()', and then check if the guest also supports the
> feature and finally updates the field in 'connect_ring()'.
>
> However, 'connect_ring()' could be called before 'xen_vbd_create()', so
> later execution of 'xen_vbd_create()' can wrongly overwrite 'true' to
> 'vbd->feature_gnt_persistent'.  As a result, 'blkback' could try to use
> 'persistent grants' feature even if the guest doesn't support the
> feature.
>
> This commit fixes the issue by moving the parameter value caching to
> 'xen_blkif_alloc()', which allocates the 'blkif'.  Because the struct
> embeds 'vbd' object, which will be used by 'connect_ring()' later, this
> should be called before 'connect_ring()' and therefore this should be
> the right and safe place to do the caching.
>
> Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
> Cc: <stable@xxxxxxxxxxxxxxx> # 5.10.x
> Signed-off-by: Maximilian Heyne <mheyne@xxxxxxxxx>
> Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
> ---
>
> Changes from v1[1]
> - Avoid the behavioral change[2]
> - Rebase on latest xen/tip/linux-next
> - Re-work by SeongJae Park <sj@xxxxxxxxxx>
> - Cc stable@
>
> [1] https://lore.kernel.org/xen-devel/20220106091013.126076-1-mheyne@xxxxxxxxx/
> [2] https://lore.kernel.org/xen-devel/20220121102309.27802-1-sj@xxxxxxxxxx/
>
>   drivers/block/xen-blkback/xenbus.c | 15 +++++++--------
>   1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index 97de13b14175..16c6785d260c 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -157,6 +157,11 @@ static int xen_blkif_alloc_rings(struct xen_blkif *blkif)
>       return 0;
>   }
>   
> +/* Enable the persistent grants feature. */
> +static bool feature_persistent = true;
> +module_param(feature_persistent, bool, 0644);
> +MODULE_PARM_DESC(feature_persistent, "Enables the persistent grants feature");
> +
>   static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>   {
>       struct xen_blkif *blkif;
> @@ -181,6 +186,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>       __module_get(THIS_MODULE);
>       INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
>   
> +     blkif->vbd.feature_gnt_persistent = feature_persistent;
> +
>       return blkif;
>   }
>   
> @@ -472,12 +479,6 @@ static void xen_vbd_free(struct xen_vbd *vbd)
>       vbd->bdev = NULL;
>   }
>   
> -/* Enable the persistent grants feature. */
> -static bool feature_persistent = true;
> -module_param(feature_persistent, bool, 0644);
> -MODULE_PARM_DESC(feature_persistent,
> -             "Enables the persistent grants feature");
> -
>   static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
>                         unsigned major, unsigned minor, int readonly,
>                         int cdrom)
> @@ -520,8 +521,6 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
>       if (bdev_max_secure_erase_sectors(bdev))
>               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;

--
Regards,

Oleksandr Tyshchenko