linux-kernel - Re: [Xen-devel] [PATCH v2] xen-blkfront: remove type check from blkfront_setup

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Date:	Tue, 14 Jan 2014 15:17:04 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Olaf Hering <olaf@...fle.de>
CC:	David Vrabel <david.vrabel@...rix.com>,
	<linux-kernel@...r.kernel.org>, <boris.ostrovsky@...cle.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Jan Beulich <JBeulich@...e.com>, <xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] [PATCH v2] xen-blkfront: remove type check from blkfront_setup_discard

On 14/01/14 14:53, Olaf Hering wrote:
> On Mon, Jan 13, David Vrabel wrote:
> 
>> Can we have a patch to blkif.h that clarifies this?
> 
> What about this change?
> 
> diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h
> index 84eb7fd..56e2faa 100644
> --- a/xen/include/public/io/blkif.h
> +++ b/xen/include/public/io/blkif.h
> @@ -194,6 +194,7 @@
>   * discard-secure
>   *      Values:         0/1 (boolean)
>   *      Default Value:  0
> + *      Notes:          10
>   *
>   *      A value of "1" indicates that the backend can process BLKIF_OP_DISCARD
>   *      requests with the BLKIF_DISCARD_SECURE flag set.
> @@ -323,9 +324,10 @@
>   *     For full interoperability, block front and backends should publish
>   *     identical ring parameters, adjusted for unit differences, to the
>   *     XenStore nodes used in both schemes.
> - * (4) Devices that support discard functionality may internally allocate
> - *     space (discardable extents) in units that are larger than the
> - *     exported logical block size.
> + * (4) Devices that support discard functionality may internally allocate space
> + *     (discardable extents) in units that are larger than the exported logical
> + *     block size. The properties discard-granularity and discard-alignment may
> + *     be present if the backing device has such requirments.

Clarify that both discard-granularity and discard-alignment must be
present if non-sector-sized granularity is required. e.g.,

"If the backing device has such discardable extents the backend must
provide both discard-granularity and discard-alignment."

You find it useful to add these recommendations:

"Backends supporting discard should include discard-granularity and
discard-alignment even if it supports discarding individual sectors.
Frontends should assume discard-aligment == 0 and discard-granularity ==
sector size if these keys are missing."

>   * (5) The discard-alignment parameter allows a physical device to be
>   *     partitioned into virtual devices that do not necessarily begin or
>   *     end on a discardable extent boundary.
> @@ -344,6 +346,8 @@
>   *     grants that can be persistently mapped in the frontend driver, but
>   *     due to the frontent driver implementation it should never be bigger
>   *     than RING_SIZE * BLKIF_MAX_SEGMENTS_PER_REQUEST.
> + *(10) The discard-secure property may be present and will be set to 1 if the
> + *     backing device supports secure discard.
>   */

David
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