On Wed, Mar 31, 2010 at 11:57:46AM -0700, Eric W. Biederman wrote: > 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. > You mean like initialize the device table so that all devices are default disabled on boot, and then selectively enable them (perhaps during a device_attach)? I can give that a spin. Neil > Eric > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo at vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
