Re: [PATCH] amd iommu: force flush of iommu prior during shutdown

[Vivek Goyal <vgoyal@redhat.com>]

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@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