On Wed, Mar 31, 2010 at 09:13:11PM -0400, Neil Horman wrote: > On Wed, Mar 31, 2010 at 02:25:35PM -0700, Chris Wright wrote: > > * Neil Horman (nhorman at tuxdriver.com) wrote: > > > Flush iommu during shutdown > > > > > > When using an iommu, its possible, if a kdump kernel boot follows a primary > > > kernel crash, that dma operations might still be in flight from the previous > > > kernel during the kdump kernel boot. This can lead to memory corruption, > > > crashes, and other erroneous behavior, specifically I've seen it manifest during > > > a kdump boot as endless iommu error log entries of the form: > > > AMD-Vi: Event logged [IO_PAGE_FAULT device=00:14.1 domain=0x000d > > > address=0x000000000245a0c0 flags=0x0070] > > > > We've already fixed this problem once before, so some code shift must > > have brought it back. Personally, I prefer to do this on the bringup > > path than the teardown path. Besides keeping the teardown path as > > simple as possible (goal is to get to kdump kernel asap), there's also > > reason to competely flush on startup in genernal in case BIOS has done > > anything unsavory. > > > Chris, > Can you elaborate on what you did with the iommu to make this safe? It > will save me time digging through the history on this code, and help me > understand better whats going on here. > > I was starting to think that we should just leave the iommu on through a kdump, > and re-construct a new page table based on the old table (filtered by the error > log) on kdump boot, but it sounds like a better solution might be in place. > Hi Neil, Is following sequence possible. - In crashed kernel, take away the write permission from all the devices. Mark bit 62 zero for all devices in device table. - Leave the iommu on and let the device entries be valid in kdump kernel so that any in-flight dma does not become pass through (which can cause more damage and corrupt kdump kernel). - During kdump kernel initialization, load a new device table where again all the devices don't have write permission. looks like by default we create a device table with all bits zero except DEV_ENTRY_VALID and DEV_ENTRY_TRANSLATION bit. - Reset the device where we want to setup any dma or operate on. - Allow device to do DMA/write. So by default all the devices will not be able to do write to memory and selective devices are given access only after a reset. I am not sure what are the dependencies for loading a new device table in second kernel. If it requires disabling the IOMMU, then we leave a window where in-flight dma will become passthrough and has the potential to corrupt kdump kernel. Thanks Vivek
