Return-Path: Received: from smtp.opengridcomputing.com ([72.48.136.20]:49618 "EHLO smtp.opengridcomputing.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753544AbbGWTDF (ORCPT ); Thu, 23 Jul 2015 15:03:05 -0400 From: "Steve Wise" To: "'Hefty, Sean'" , "'Christoph Hellwig'" Cc: "'Sagi Grimberg'" , "'Steve Wise'" , "'Jason Gunthorpe'" , "'Tom Talpey'" , "'Doug Ledford'" , , , , , , , , , , "'Oren Duer'" References: <20150708190842.GB11740@obsidianresearch.com> <20150708203205.GA21847@infradead.org> <20150709000337.GE16812@obsidianresearch.com> <559EF332.7060103@redhat.com> <20150709225306.GA30741@obsidianresearch.com> <559FC710.1050307@talpey.com> <20150710161108.GA19042@obsidianresearch.com> <55A24571.60902@dev.mellanox.co.il> <00e201d0be6a$e49bc910$add35b30$@opengridcomputing.com> <20150714194512.GA25887@infradead.org> <00f901d0be6f$70c96b00$525c4100$@opengridcomputing.com> <1828884A29C6694DAF28B7E6B8A82373A901C9A5@ORSMSX109.amr.corp.intel.com> In-Reply-To: <1828884A29C6694DAF28B7E6B8A82373A901C9A5@ORSMSX109.amr.corp.intel.com> Subject: RE: [PATCH V3 1/5] RDMA/core: Transport-independent access flags Date: Thu, 23 Jul 2015 14:03:10 -0500 Message-ID: <009e01d0c57a$37d5b9e0$a7812da0$@opengridcomputing.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Sender: linux-nfs-owner@vger.kernel.org List-ID: > -----Original Message----- > From: Hefty, Sean [mailto:sean.hefty@intel.com] > Sent: Thursday, July 23, 2015 1:53 PM > To: Steve Wise; 'Christoph Hellwig' > Cc: 'Sagi Grimberg'; 'Steve Wise'; 'Jason Gunthorpe'; 'Tom Talpey'; 'Doug Ledford'; sagig@mellanox.com; ogerlitz@mellanox.com; > roid@mellanox.com; linux-rdma@vger.kernel.org; eli@mellanox.com; target-devel@vger.kernel.org; linux-nfs@vger.kernel.org; > trond.myklebust@primarydata.com; bfields@fieldses.org; 'Oren Duer' > Subject: RE: [PATCH V3 1/5] RDMA/core: Transport-independent access flags > > > 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? :) > > This may be a nit, but IMO, the use of the term 'rkey' versus 'stag' matters. They convey different ways of finding a data buffer. For > example, do you locate a buffer using the stag, then verify that the offset + length fits into the target buffer? Yes. HW always uses the stag to locate a record that contains the stag state (valid or invalid), the access flags, the 8b key, the va_base, length, PBL describing the host pages, etc. HW validates all that before using the buffer. NOTE: An stag of 0 is the special local-dma-lkey which HW treats differently: If the stag is 0, then the address in the SGE is the bus/dma address itself and no lookup of a MR/PBL/etc is needed. Stag 0 can ONLY be used by kernel users and MUST never be accepted/used from an ingress packet and MUST never be emitted on the wire in a READ or WRITE. > Or do you locate the buffer > by address, then verify that the key matches? > This is never done. > Consider if we allow an app to specify the rkey/stag, or reference the buffer using an offset, rather than a virtual address. > > This seems to be part of the difference between an lkey and an rkey.