On 17.05.2011, at 11:35, Benjamin Herrenschmidt wrote: > On Tue, 2011-05-17 at 11:31 +0200, Alexander Graf wrote: >> On 17.05.2011, at 11:11, Benjamin Herrenschmidt wrote: >> >>> On Tue, 2011-05-17 at 10:01 +0200, Alexander Graf wrote: >>>> I'm not sure I fully understand how this is supposed to work. If the >>>> tables are kept inside the kernel, how does userspace get to know >>>> where to DMA to? >>> >>> The guest gets a dma range from the device-tree which is the range of >>> device-side dma addresses it can use that correspond to the table. >>> >>> The guest kernel uses the normal linux iommu space allocator to allocate >>> space in that region and uses H_PUT_TCE to populate the corresponding >>> table entries. >>> >>> This is the same interface that is used for "real" iommu's with PCI >>> devices btw. >> >> I'm still slightly puzzled here :). IIUC the main point of an IOMMU is for the kernel >> to change where device accesses actually go to. So device DMAs address A, goes through >> the IOMMU, in reality accesses address B. > > Right :-) > >> Now, how do we tell the devices implemented in qemu that they're supposed to DMA to >> address B instead of A if the mapping table is kept in-kernel? > > Oh, bcs qemu mmaps the table :-) That's the piece to the puzzle I was missing. Please document that interface properly - it needs to be rock stable :) Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
