Re: [PATCH] xen-blkfront: remove type check from blkfront_setup_discard

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 01/10/2014 04:37 PM, Olaf Hering wrote:
> On Fri, Jan 10, Boris Ostrovsky wrote:
>
>> If the call below fails, is it safe to continue using discard feature? At
>> the least, are discard_granularity and discard_alignment guaranteed to have
>> sane/safe values?
> Its up to the toolstack to provide sane values. In the worst case
> discard fails. In this specific case the three values are optional, so
> the calls can fail. I do not know what happens if the backend device
> actually needs the values, but the frontend can not send proper discard
> requests. Hopefully it will not damage the hardware..

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.

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.

I think we should at clear feature_discard and print an error in the log 
if *either* of xenbus_gather() calls fail.


-boris