Re: [PATCH 0/6] IOMMU/DMA map_resource support for peer-to-peer

[Mark Hounschell <markh-n2QNKt385d+sTnJN9+BGXg@public.gmane.org>]

On 05/08/2015 04:21 PM, Konrad Rzeszutek Wilk wrote:
> On Fri, May 01, 2015 at 01:32:12PM -0500, wdavis-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org wrote:
>> From: Will Davis <wdavis-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
>>
>> Hi,
>>
>> This patch series adds DMA APIs to map and unmap a struct resource to and from
>> a PCI device's IOVA domain, and implements the AMD, Intel, and nommu versions
>> of these interfaces.
>>
>> This solves a long-standing problem with the existing DMA-remapping interfaces,
>> which require that a struct page be given for the region to be mapped into a
>> device's IOVA domain. This requirement cannot support peer device BAR ranges,
>> for which no struct pages exist.
>>
>> The underlying implementations of map_page and map_sg convert the struct page
>> into its physical address anyway, so we just need a way to route the physical
>> address of the BAR region to these implementations. The new interfaces do this
>> by taking the struct resource describing a device's BAR region, from which the
>> physical address is derived.
>>
>> The Intel and nommu versions have been verified on a dual Intel Xeon E5405
>> workstation. I'm in the process of obtaining hardware to test the AMD version
>> as well. Please review.
>
> Does it work if you boot with 'iommu=soft swiotlb=force' which will mandate
> an strict usage of the DMA API?
>

PCIe peer2peer is borked on all motherboards I've tried. Only writes are 
possible. Reads are not supported. I suppose if you have a platform with 
only PCI and an IOMMU this would be very useful. Without both read and 
write PCIe peer2peer support, this seems unnecessary.

Mark