Re: blkfront problem in pvops kernel when barriers enabled

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 09/06/2011 06:47 PM, Konrad Rzeszutek Wilk wrote:
> (on 3.1rc2) Looking to xenstore now there is 'feature-flush-cache=1' and
> no 'feature-barrier'. So it is ok.
> <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'.

Does that mean that older guests which don't understand flush-cache will
be left with no way to force writes to stable storage?  Seems to me that
even if the backend would prefer flush-cache, it should also advertise
barriers.

However, that raises the question of how to express the preferred
mechanism if multiple are available.  You could assume that flush-cache
is always preferred if available, but that's pretty clunky.

    J