> -----Original Message----- > From: Hefty, Sean [mailto:sean.hefty@xxxxxxxxx] > 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@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 > > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
