Alignment Issue with Direct IO to NVMe Drive

[Matthew Wilcox]

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 at linux.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.