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SubjectRe: [PATCH 2/2] virtio_ring: Use DMA API if memory is encrypted

On 2019/10/15 下午3:35, Christoph Hellwig wrote:
> On Fri, Oct 11, 2019 at 06:25:19PM -0700, Ram Pai wrote:
>> From: Thiago Jung Bauermann <>
>> Normally, virtio enables DMA API with VIRTIO_F_IOMMU_PLATFORM, which must
>> be set by both device and guest driver. However, as a hack, when DMA API
>> returns physical addresses, guest driver can use the DMA API; even though
>> device does not set VIRTIO_F_IOMMU_PLATFORM and just uses physical
>> addresses.
> Sorry, but this is a complete bullshit hack. Driver must always use
> the DMA API if they do DMA, and if virtio devices use physical addresses
> that needs to be returned through the platform firmware interfaces for
> the dma setup. If you don't do that yet (which based on previous
> informations you don't), you need to fix it, and we can then quirk
> old implementations that already are out in the field.
> In other words: we finally need to fix that virtio mess and not pile
> hacks on top of hacks.

I agree, the only reason for IOMMU_PLATFORM is to make sure guest works
for some old and buggy qemu when vIOMMU is enabled which seems useless
(note we don't even support vIOMMU migration in that case).


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