> -----Original Message----- > From: 'Christoph Hellwig' [mailto:hch@xxxxxxxxxxxxx] > Sent: Tuesday, July 14, 2015 2:45 PM > To: Steve Wise > Cc: 'Sagi Grimberg'; 'Steve Wise'; 'Jason Gunthorpe'; 'Tom Talpey'; 'Doug Ledford'; 'Christoph Hellwig'; sagig@xxxxxxxxxxxx; > ogerlitz@xxxxxxxxxxxx; roid@xxxxxxxxxxxx; linux-rdma@xxxxxxxxxxxxxxx; eli@xxxxxxxxxxxx; target-devel@xxxxxxxxxxxxxxx; linux- > nfs@xxxxxxxxxxxxxxx; trond.myklebust@xxxxxxxxxxxxxxx; bfields@xxxxxxxxxxxx; 'Oren Duer' > Subject: Re: [PATCH V3 1/5] RDMA/core: Transport-independent access flags > > On Tue, Jul 14, 2015 at 02:25:50PM -0500, Steve Wise wrote: > > if (device_supports_fastreg && device_supports_signature) > > use FRMR > > else > > use DMAMR > > > > Shouldn't we just recode it this way? > > > > if (device_supports_fastreg) > > use FRMR > > else > > use DMAMR > > How does IB_DEVICE_LOCAL_DMA_LKEY (and ->local_dma_lkey) play into > this? Seems like that should be the preferred option if supported. > The local_dma_lkey can be used in any rdma sge that requires an lkey. It is supported for kernel-mode only. So if you're only ever going to use the lkey for IO, you really don't need a DMA_MR at all, unless you want to somehow partition up your work load by protection domain. But I claim for lkeys, the PD doesn't really protect anything since the remote peers can't use it anyway. > Interestingly enough various iWarp driver seem to support this option, > what's the story behind that? The (to me surprising) conclusion on > the list was that iWarp would always need a memory regireations that > also allows remove writes even for lkeys, but from looking at the > users of IB_DEVICE_LOCAL_DMA_LKEY / local_dma_lkey that seem to > prefer that over creating a MR. There is confusion about lkeys and rkeys with regard to iWARP. In the iWARP verbs, there is no distinction between an lkey and rkey: they are the same key, called a Steering Tag or STAG. When you create a MR, the lkey == rkey == STAG for iwarp transports. Somewhat related, but really a different issue, is that SGEs that are the target of a read need REMOTE_WRITE access flags on their STAG for iWARP. Clear as mud? :) -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
