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

Jens Axboe
2014-06-11T16:55:53Z

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.