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SubjectRe: [PATCH] s390: protvirt: virtio: Refuse device without IOMMU
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On 2020-06-12 15:45, Mauricio Tavares wrote:
> On Wed, Jun 10, 2020 at 12:32 PM Pierre Morel <pmorel@linux.ibm.com> wrote:
>>
>> Protected Virtualisation protects the memory of the guest and
>> do not allow a the host to access all of its memory.
>>
>> Let's refuse a VIRTIO device which does not use IOMMU
>> protected access.
>>
> Stupid questions:

not stupid at all. :)

>
> 1. Do all CPU families we care about (which are?) support IOMMU? Ex:
> would it recognize an ARM thingie with SMMU? [1]

In Message-ID: <6356ba7f-afab-75e1-05ff-4a22b88c610e@linux.ibm.com>
(as answer to Jason) I modified the patch and propose to take care of
this problem by using force_dma_unencrypted() inside virtio core instead
of a S390 specific test.

If we use force_dma_unencrypted(dev) to check if we must refuse a device
without the VIRTIO_F_IOMMU_PLATFORM feature, we are safe:
only architectures defining CONFIG_ARCH_HAS_FORCE_DMA_UNENCRYPTED will
have to define force_dma_unencrypted(dev), and they can choose what to
do by checking the architecture functionalities and/or the device.

> 2. Would it make sense to have some kind of
> yes-I-know-the-consequences-but-I-need-to-have-a-virtio-device-without-iommu-in-this-guest
> flag?

Yes, two ways:

Never refuse a device without VIRTIO_F_IOMMU_PLATFORM, by not defining
CONFIG_ARCH_HAS_FORCE_DMA_UNENCRYPTED or by always return 0 in
force_dma_unencrypted()

have force_dma_unencrypted() selectively answer by checking the device
and/or architecture state.

>
...snip...
>>
>
> [1] https://developer.arm.com/architectures/system-architectures/system-components/system-mmu-support
>

Regards,
Pierre


--
Pierre Morel
IBM Lab Boeblingen

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