From mboxrd@z Thu Jan  1 00:00:00 1970
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: blkfront problem in pvops kernel when barriers enabled
Date: Wed, 7 Sep 2011 13:41:58 -0400
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To: Jan Beulich <JBeulich@suse.com>
Cc: "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, Marek Marczykowski <marmarek@mimuw.edu.pl>
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> >> <scratches head>
> >> 
> >> I can only think of 2.6.38-3 XenOLinux doing it - and it is a bug
> >> to do it. It really ought to _not_ advertise 'feature-barrier' and
> >> instead advertise 'feature-flush-cache'.
> > 
> > Indeed, I see that I added feature-flush-cache support to the frontend
> > back then, but neglected to do so for the backend. Partly perhaps
> > because I'm not much of a (block, network, ...) driver person...
> > 
> > However, what I'm not understanding with dropping feature-barrier
> > support from the backend - how do you deal with old frontends
> > wanting to use barriers? I'm currently converting them into

Just not supporting them. I know it is incredibly bad to do so - but
I have not had a chance to write the code to emulate the 'feature-barrier'
correctly.

> > WRITE_FLUSH_FUA operations in the backend as a (hopefully) best
> > effort approach.

I am not sure. I need to run blktrace|blkparse to make sure it does the
right think as compared to a WRITE_BARRIER. Lets ask Christopher Hellwig - he
knows a lot of this.

> 
> Also I notice you're using WRITE_ODIRECT - what's the background
> of that?

Ah, http://git.drbd.org/linux-2.6-drbd.git/?p=linux-2.6-drbd.git;a=commit;h=013c3ca184851078b9c04744efd4d47e52c6ecf8


> 
> Thanks, Jan