linux-kernel - Re: [PATCH v7] NVMe: conversion to blk-mq

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Date:	Wed, 11 Jun 2014 10:54:52 -0600
From:	Jens Axboe <axboe@...com>
To:	Matthew Wilcox <willy@...ux.intel.com>,
	Keith Busch <keith.busch@...el.com>
CC:	Matias Bjørling <m@...rling.me>,
	"sbradshaw@...ron.com" <sbradshaw@...ron.com>,
	"tom.leiming@...il.com" <tom.leiming@...il.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>
Subject: Re: [PATCH v7] NVMe: conversion to blk-mq

On 06/10/2014 03:33 PM, Matthew Wilcox wrote:
> On Tue, Jun 10, 2014 at 03:21:18PM -0600, Keith Busch wrote:
>> Yeah, nvme_setup_prps is probably the least readable code in this driver.
>> Maybe some comments are in order here...
>>
>> There are two rules for an SGL to be mappable to a PRP:
>>
>> 1. Every element must have zero page offset, except the first.
>>
>> 2. Every element must end on a page boundary, except the last.
> 
> Or to put it another way, NVMe PRPs only support I/Os that describe a
> single range of virtual memory.

OK, so essentially any single request must be a virtually contig piece
of memory. Is there any size limitations to how big this contig segment
can be?

I think this is unique requirement, at least I haven't seen other pieces
of hardware have it. But it would be pretty trivial to add a setting to
limit merges based on virtually contig, similarly to what is done for
number of physical segments.

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