On Wed, May 08, 2019 at 02:06:05PM +0000, Bernard Metzler wrote: > > >To: "Bernard Metzler" <BMT@xxxxxxxxxxxxxx> > >From: "Jason Gunthorpe" <jgg@xxxxxxxx> > >Date: 05/08/2019 03:08PM > >Cc: "Leon Romanovsky" <leon@xxxxxxxxxx>, linux-rdma@xxxxxxxxxxxxxxx > >Subject: Re: [PATCH v8 02/12] SIW main include file > > > >On Wed, May 08, 2019 at 08:07:59AM +0000, Bernard Metzler wrote: > >> >> >> Memory access keys and QP IDs are generated as random > >> >> >> numbers, since both are exposed to the application. > >> >> >> Since XArray is not designed for sparsely distributed > >> >> >> id ranges, I am still in favor of IDR for these two > >> >> >> resources. > >> > > >> >IDR and xarray have identical underlying storage so this is > >nonsense > >> > > >> >No new idr's or radix tree users will be accepted into rdma.... > >Use > >> >xarray > >> > > >> Sounds good to me! I just came across that introductory video from > >Matthew, > >> where he explicitly stated that xarray will be not very efficient > >if the > >> indices are not densely clustered. But maybe this is all far beyond > >the > >> 24bits of index space a memory key is in. So let me drop that IDR > >thing > >> completely, while handling randomized 24 bit memory keys. > > > >xarray/idr is a poor choice to store highly unclustered random data > > > >I'm not sure why this is a problem, shouldn't the driver be in > >control > >of mkey assignment? Just use xa_alloc_cyclic and it will be > >sufficiently clustered to be efficient. > > > > It is a recommendation to choose a hard to predict memory > key (to make it hard for an attacker to guess it). From > RFC 5040, sec 8.1.1: > > An RNIC MUST choose the value of STags in a way difficult to > predict. It is RECOMMENDED to sparsely populate them over the > full available range. > > Since I did not want to roll my own bug-prone key based lookup, > I chose idr. If you tell me xarray is just as inefficient as > idr for sparse index distributions, I'll take xarray. Yah, this probably wants to be a RB tree or some other data structure.. But you can leave it as xarray it just wastes memory. Jason
