On Thu, Apr 01, 2010 at 11:02:03AM -0400, Vivek Goyal wrote: > On Thu, Apr 01, 2010 at 08:53:04AM -0400, Neil Horman wrote: > > On Wed, Mar 31, 2010 at 10:24:18PM -0400, Vivek Goyal wrote: > > > 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. > > > > > I think this is possible, but I'm a bit concerned with how some devices will > > handle a reset. For instance, what will happen to an HBA or a disk, if we reset > > it as the module is loading? Is that safe? > > I think we need to reset devices in driver if "reset_devices" is set. So > we will not reset these during normal boot. > > Regarding being safe, I don't know. I am assuming that driver knows (or > need to know), how to reset device safely while driver is initializing. > That's the whole assumption kdump is built on, that once driver is > initializing, it will first reset the device (if reset_devices is set), so > that chances of device working properly in second kernel increase. > Yes, I agree, I was more just asking is it safe to unilaterally reset devices during boot? I suppose it is, but I'm not entirely sure Neil > Vivek >
