Re: [RFC] block layer support for DMA IOMMU bypass mode

[James Bottomley <James.Bottomley@steeleye.com>]

On Tue, 2003-07-01 at 14:59, Alex Williamson wrote:
>    The thing that's got me concerned about this is that it allows
> for sg lists that contains both entries that the block layer
> expects will be mapped into the iommu and ones that it expects
> to bypass.  I don't like the implications of parsing through
> sg lists looking for bypass-able and non-bypass-able groupings.
> This seems like a lot more overhead than we have now and the
> complexity of merging partially bypass-able scatterlists seems
> time consuming.
> 
>    The current ia64 sba_iommu does a quick and dirty sg bypass
> check.  If the device can dma to any memory address, the entire
> sg list is bypassed.  If not, the entire list is coalesced and
> mapped by the iommu.  The idea being that true performance
> devices will have 64bit dma masks and be able to quickly bypass.
> Everything else will at least get the benefit of coalescing
> entries to make more efficient dma.  The coalescing is a bit
> simpler since it's the entire list as well.  With this proposal,
> we'd have to add a lot of complexity to partially bypass sg
> lists.  I don't necessarily see that as a benefit.  Thanks,

But if that's all you want, you simply set the BIO_VMERGE_BYPASS_MASK to
the full u64 set bitmask.  Then it will only turn off virtual merging
for devices that have a fully set dma_mask, and your simple test will
work.

James