Re: [PATCH RFC 4/4] xen-block: introduce a new request type to unmap grants

["Roger Pau Monné" <roger.pau@citrix.com>]

On 08/07/13 21:41, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 08, 2013 at 03:03:27PM +0200, Roger Pau Monne wrote:
>> Right now blkfront has no way to unmap grant refs, if using persistent
>> grants once a grant is used blkfront cannot assure if blkback will
>> have this grant mapped or not. To solve this problem, a new request
>> type (BLKIF_OP_UNMAP) that allows requesting blkback to unmap certain
>> grants is introduced.
> 
> I don't think this is the right way of doing it. It is a new operation
> (BLKIF_OP_UNMAP) that has nothing to do with READ/WRITE. All it is
> is just some way for the frontend to say: unmap this grant if you can.
> 
> As such I would think a better mechanism would be to have a new
> grant mechanism that can say: 'I am done with this grant you can
> remove it' - that is called to the hypervisor. The hypervisor
> can then figure out whether it is free or not and lazily delete it.
> (And the guest would be notified when it is freed).

I would prefer not to involve the hypervisor in persistent grants, this
is something between the frontends and the backends. The hypervisor
already provides the basic operations (map/unmap), IMHO there's no need
to add more logic to the hypervisor itself.

I agree that it would be better to have a generic way to request a
backend to unmap certain grants, but so far this seems like the best
solution.

> 
> I would presume that this problem would also exist with netback/netfront
> if it started using persisten grants, right?

I'm not sure of that, it depends on the number of persistent grants
netfront/netback use, in the block case we need this operation because
of indirect descriptors, but netfront/netback might not suffer from this
problem if the maximum number of grants they use is relatively small.