linux-kernel - Re: [PATCH 1/2] x86/amd-iommu: enable iommu before attaching devices

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Message-ID: <20100403173836.GP24846@8bytes.org>
Date:	Sat, 3 Apr 2010 19:38:36 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Neil Horman <nhorman@...hat.com>,
	Neil Horman <nhorman@...driver.com>, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, hbabu@...ibm.com,
	iommu@...ts.linux-foundation.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 1/2] x86/amd-iommu: enable iommu before attaching
	devices

On Fri, Apr 02, 2010 at 11:59:32AM -0400, Vivek Goyal wrote:
> 1. kernel crashes, we leave IOMMU enabled.

True for everything except gart and amd iommu.

> 	a. So during this small window when iommu is disabled and we enable
> 	   it back, any inflight DMA will passthrough possibly to an
> 	   unintended physical address as translation is disabled and it
> 	   can corrupt the kdump kenrel.

Right.

> 	b. Even after enabling the iommu, I guess we will continue to
> 	   use cached DTE, and translation information to handle any
> 	   in-flight DMA. The difference is that now iommus are enabled
> 	   so any in-flight DMA should go to the address as intended in
> 	   first kenrel and should not corrupt anything.

Right.

> 
> 3. Once iommus are enabled again, we allocated and initilize protection
>    domains. We attach devices to domains. In the process we flush the
>    DTE, PDE and IO TLBs.
> 
> 	c. Looks like do_attach->set_dte_entry(), by default gives write
> 	   permission (IW) to all the devices. I am assuming that at
> 	   this point of time translation is enabled and possibly unity
> 	   mapped.

No, The IW bit in the DTE must be set because all write permission bits
(DTE and page tabled) are ANDed to determine if a device can write to a
particular address. So as long as the paging mode is unequal to zero the
hardware will walk the page-table first to find out if the device has
write permission. With paging mode == 0 your statement about read-write
unity-mapping is true. This is used for a pass-through domain (iommu=pt)
btw.

	Joerg

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