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

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Date:	Thu, 1 Apr 2010 11:02:03 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Neil Horman <nhorman@...hat.com>
Cc:	Neil Horman <nhorman@...driver.com>, joerg.roedel@....com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Chris Wright <chrisw@...s-sol.org>, hbabu@...ibm.com,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] amd iommu: force flush of iommu prior during shutdown

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@...driver.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.

Vivek

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