Re: Alignment Issue with Direct IO to NVMe Drive

[Laine Walker-Avina]

On Tue, Nov 27, 2012 at 1:39 PM, Matthew Wilcox <willy@xxxxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxx> 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.

There is no mention of IOMMU in dmesg on a fresh boot. The processor
in the test system I'm using is an i5-2400 (Sandy Bridge).

-Laine
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