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

[Joerg Roedel <joro@8bytes.org>]

On Thu, Apr 01, 2010 at 01:11:49PM -0400, Neil Horman wrote:
> On Thu, Apr 01, 2010 at 05:56:43PM +0200, Joerg Roedel wrote:

> > The possible fix will be to enable the hardware earlier in the
> > initialization path.
> > 
> That sounds like a reasonable theory, I'll try hack something together
> shortly.

Great. So the problem might be already fixed when I am back in the
office ;-)

> > This would only prevent possible data corruption. When the IOMMU is off
> > the devices will not get a target abort but will only write to different
> > physical memory locations. The window where a target abort can happen
> > starts when the kdump kernel re-enables the IOMMU and ends when the new
> > driver for that device attaches. This is a small window but there is not
> > a lot we can do to avoid this small time window.
> > 
> Can you explain this a bit further please?  From what I read, when the iommu is
> disabled, AIUI it does no translations.  That means that any dma addresses which
> the driver mapped via the iommu prior to a crash that are stored in devices will
> just get strobed on the bus without any translation.  If those dma address do
> not lay on top of any physical ram, won't that lead to bus errors, and
> transaction aborts?  Worse, if those dma addresses do lie on top of real
> physical addresses, won't we get corruption in various places?  Or am I missing
> part of how that works?

Hm, the device address may not be a valid host physical address, thats
true. But the problem with the small time-window when the IOMMU hardware
is re-programmed from the kdump kernel still exists.
I need to think about other possible side-effects of leaving the IOMMU
enabled on shutdown^Wboot into a kdump kernel.

	Joerg