linux-kernel - Re: [PATCH] iommu: Include MSI susceptibility to DMA in creating iommu groups

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Message-ID: <20111120120043.GB20535@8bytes.org>
Date:	Sun, 20 Nov 2011 13:00:43 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Joerg Roedel <Joerg.Roedel@....com>, linux-pci@...r.kernel.org,
	iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu: Include MSI susceptibility to DMA in creating
	iommu groups

On Fri, Nov 18, 2011 at 09:32:36AM -0700, Alex Williamson wrote:
> I guess I fail to see the difference.  We group devices behind certain
> bridges together because we can't distinguish DMA from those devices.
> MSI presents an address window across all devices for which we
> potentially can't distinguish between any of them.

With an IOMMU the address window is per-device and not shared between
all devices. A MSI message is nothing more than a DMA write transaction
to a specific address. This message has a requestor-id so an IOMMU can
distinguish between devices. The AMD IOMMU for example uses that to
implement per-device remapping tables.

> The trouble is that interrupt remapping closing a hole in DMA isolation
> is a platform issue.  Is vfio supposed to know that on architecture foo
> we don't have such a hole and we don't need to look for interrupt
> remapping.  Or maybe that platform bar solved it differently and we need
> to instead check flag MSI_OK.  Current KVM doesn't care about this
> because it only does device assignment on x86.

>From device standpoint a MSI transaction is always a DMA memory write
to a given address range. The IOMMU-API should export a feature flag
whether it supports filtering on those transaction or not. We have that
today with the IOMMU_CAP_INTR_REMAP. I agree that the interface to get
this information is ugly because a domain is needed. But the interface
can be fixed. While doing this I suggest to rename that feature
IOMMU_CAP_INTR_ISOLATION or something like that.
VFIO can then check for this flag on module-load and refuse to load if
it is not available.

Regards,

	Joerg

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