Neil Horman <nhorman at tuxdriver.com> writes: > On Wed, Mar 31, 2010 at 11:54:30AM -0400, Vivek Goyal wrote: >> So this call amd_iommu_flush_all_devices() will be able to tell devices >> that don't do any more DMAs and hence it is safe to reprogram iommu >> mapping entries. >> > It blocks the cpu until any pending DMA operations are complete. Hmm, as I > think about it, there is still a small possibility that a device like a NIC > which has several buffers pre-dma-mapped could start a new dma before we > completely disabled the iommu, althought thats small. I never saw that in my > testing, but hitting that would be fairly difficult I think, since its literally > just a few hundred cycles between the flush and the actual hardware disable > operation. > > According to this though: > http://support.amd.com/us/Processor_TechDocs/34434-IOMMU-Rev_1.26_2-11-09.pdf > That window could be closed fairly easily, but simply disabling read and write > permissions for each device table entry prior to calling flush. If we do that, > then flush the device table, any subsequently started dma operation would just > get noted in the error log, which we could ignore, since we're abot to boot to > the kdump kernel anyway. > > Would you like me to respin w/ that modification? Disabling permissions on all devices sounds good for the new virtualization capable iommus. I think older iommus will still be challenged. I think on x86 we have simply been able to avoid using those older iommus. I like the direction you are going but please let's put this in a paranoid iommu enable routine. Eric
