On Wed, 27 Sep 2017 16:31:04 +0000 Casey Leedom <leedom@xxxxxxxxxxx> wrote: > | From: Dan Williams <dan.j.williams@xxxxxxxxx> > | Sent: Tuesday, September 26, 2017 9:10 AM > | > | On Tue, Sep 26, 2017 at 9:06 AM, Casey Leedom <leedom@xxxxxxxxxxx> > wrote: | > | From: Robin Murphy <robin.murphy@xxxxxxx> > | > | Sent: Tuesday, September 26, 2017 7:22 AM > | > |... > | > ... > | > Regardless, it seems that you agree that there's an issue with > the Intel | > I/O MMU support code with regard to the legal values > which a (struct | > scatterlist) can take on? I still can't find any > documentation for this | > and, personally, I'm a bit baffled by a > Page-oriented Scatter/Gather List | > representation where [Offset, > Offset+Length) can reside outside the Page. | > | Consider the case where the page represents a huge page, then an > | offset greater than PAGE_SIZE (up to HPAGE_SIZE) makes sense. > > Okay, but whatever the underlaying Page Size is, should [Offset, > Offset+Length) completely reside within the referenced Page? I'm just > trying to understand the Invariance Conditions which are assumed by > all of the code which processes Scatter/gather Lists ... >From my experience, in general terms each scatterlist segment represents some contiguous quantity of pages, of which sg->page is the first, while sg->length and sg->offset describe the specific bounds of that segment's data. As such, the length may certainly (and frequently does) exceed PAGE_SIZE; for the offset, it's unlikely that the producer would initially construct one greater than PAGE_SIZE instead of just pointing sg->page further forward, but it seems reasonable for it to come about if some intermediate subsystem is processing an existing list in-place (as seems to be the case with crypto here). My opinion is that this may be a slightly unusual case, but I would not consider it an illegal one. I think most DMA mapping implementations would handle it whether intentionally or not. Robin.
