On Fri, 2015-02-20 at 16:01 +1100, Benjamin Herrenschmidt wrote: > Hi Sreekanth ! > > While looking at some (unrelated) issue where mtp2sas seems to be using > 32-bit DMA instead of 64-bit DMA on some POWER platforms, I noticed this > patch which was merged as 5fb1bf8aaa832e1e9ca3198de7bbecb8eff7db9c. > > Can you confirm my understanding that you are: > > - Setting the DMA mask to 32-bit > > - Mapping pages for DMA > > - Changing the DMA mask to 64-bit > > ? > > If yes, then I don't think this is a legal thing to do and definitely > not something supported by all architectures. It might work by accident, > but there is no telling that any translation/DMA mapping provided before > a call to set_dma_mask() is still valid after that call. > > The architecture might have to completely reconfigure the iommu, for > example on some PowerPC platforms, we switch from a remapped mapping to > a direct linear map of all memory, all translations established before > the switch might be lost (it depends on the specific implementation). > > How does it work on x86 with DMAR ? Note that even on powerpc platforms where it would work because we maintain both 32-bit and 64-bit bypass windows in the device address space simultaneously, you will leak iommu entries unless you also switch back to 32-bit when freeing the 32-bit mappings... (and you would probably crash if you tried to free a 64-bit mapping while in 32-bit mode). The iommu APIs weren't designed with that "switching mask" facility in mind... Ben. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
