linux-kernel - Re: Alignment Issue with Direct IO to NVMe Drive

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Message-ID: <20121127213943.GB5100@linux.intel.com>
Date:	Tue, 27 Nov 2012 16:39:43 -0500
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Laine Walker-Avina <lwalkera@...e.org>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-nvme@...ts.infradead.org,
	linux-kernel@...r.kernel.org, lwalkera@...ron.com
Subject: Re: Alignment Issue with Direct IO to NVMe Drive

On Tue, Nov 27, 2012 at 09:47:48AM -0800, Laine Walker-Avina wrote:
> On Tue, Nov 27, 2012 at 9:05 AM, Matthew Wilcox <willy@...ux.intel.com> wrote:
> > Laine, when this occurs, what is the alignment of 'offset' in the sg
> > entry you're looking at?  If userspace is passing in an unaligned address,
> > I don't think there's anything we do to try to align it.
> 
> I thought this was the case as well, but it appears to be aligned
> looking at the request in the user space program (fio using libaio in
> this case) before the IO is submitted.

OK, so we have an aligned virtual address being translated into an
unaligned DMA address.  I have a suspicion ... are you using the swiotlb,
or are you using a real IOMMU?  Grep dmesg for 'IOMMU' if you're not sure.
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