Re: [PATCH] iommu: Include MSI susceptibility to DMA in creatingiommu groups

[Kai Huang]

On Sun, Nov 20, 2011 at 8:00 PM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
> 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.

In my understanding the Interrupt Remapping provides device isolation
from the security perspective. The VFIO framework is designed to
"expose direct device access to userspace, in a secure, IOMMU
protected environment" (copied from vfio.txt, I am not familiar with
vfio). Without Interrupt Remapping, an spurious interrupt issued by
user space driver may crash other groups or the whole system entirely.
>From this point, I think it is reasonable to disable group without
Interrupt Remapping support in IOMMU, or at least give user a notice.

-cody

>
>> 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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