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Subject: Re: [PATCH] xen-blkfront: remove type check from blkfront_setup_discard
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Date: Mon, 13 Jan 2014 11:24:36 -0500
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>, Olaf Hering <olaf@aepfle.de>
CC: xen-devel@lists.xen.org, linux-kernel@vger.kernel.org,
        david.vrabel@citrix.com
Message-ID: <1a76cddb-cb54-4483-9ab2-a750ef0d9962@email.android.com>
Sender: linux-kernel-owner@vger.kernel.org

Boris Ostrovsky <boris.ostrovsky@oracle.com> wrote:
>On 01/13/2014 04:30 AM, Olaf Hering wrote:
>> On Fri, Jan 10, Boris Ostrovsky wrote:
>>
>>> I don't know discard code works but it seems to me that if you pass,
>for
>>> example,  zero as discard_granularity (which may happen if
>xenbus_gather()
>>> fails) then blkdev_issue_discard() in the backend will set
>granularity to 1
>>> and continue with discard. This may not be what the the guest admin
>>> requested. And he won't know about this since no error message is
>printed
>>> anywhere.
>> If I understand the code using granularity/alignment correctly, both
>are
>> optional properties. So if the granularity is just 1 it means byte
>> ranges, which is fine if the backend uses FALLOC_FL_PUNCH_HOLE. Also
>> both properties are not admin controlled, for phy the blkbk drivers
>just
>> passes on what it gets from the underlying hardware.
>>
>>> Similarly, if xenbug_gather("discard-secure") fails, I think the
>code will
>>> assume that secure discard has not been requested. I don't know what
>>> security implications this will have but it sounds bad to me.
>> There are no security implications, if the backend does not advertise
>it
>> then its not present.
>
>Right. But my questions was what if the backend does advertise it and 
>wants the frontent to use it but xenbus_gather() in the frontend fails.
>
>Do we want to silently continue without discard-secure? Is this safe?
>

Yes
>
>-boris
>
>>
>> After poking around some more it seems that blkif.h is the spec, it
>does
>> not say anything that the three properties are optional. Also the
>> backend drivers in sles11sp2 and mainline create all three properties
>> unconditionally. So I think a better change is to expect all three
>> properties in the frontend. I will send another version of the patch.
>>
>>
>> Olaf


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