Re: [PATCH 0/3] Fix virtio-blk issue with SWIOTLB

[Christoph Hellwig]

On Mon, Jan 14, 2019 at 01:20:45PM -0500, Michael S. Tsirkin wrote:
> I don't think so - the issue is really that DMA API does not yet handle
> the SEV case 100% correctly. I suspect passthrough devices would have
> the same issue.

The DMA API handles the SEV case perfectly.  Its just that virtio_blk
supports huge segments that virtio does not generally support, but that
is not related to SEV in any way.

> In fact whoever sets IOMMU_PLATFORM is completely unaffected by
> Christoph's pet peeve.

No, the above happens only when we set IOMMU_PLATFORM.

> Christoph is saying that !IOMMU_PLATFORM devices should hide the
> compatibility code in a special per-device DMA API implementation.
> Which would be fine especially if we can manage not to introduce a bunch
> of indirect calls all over the place and hurt performance.  It's just
> that the benefit is unlikely to be big (e.g. we can't also get rid of
> the virtio specific memory barriers) so no one was motivated enough to
> work on it.

No.  The problem is that we still haven't fully specified what
IOMMU_PLATFORM and !IOMMU_PLATFORM actually mean.  Your
"ACCESS_PLATFORM/ORDER_PLATFORM" commit in the virtio-spec repo
improves it a little bit, but it is still far from enough.

As a start VIRTIO_F_ORDER_PLATFORM and VIRTIO_F_ACCESS_PLATFORM
absolutely MUST be set for hardware implementations.  Otherwise said
hardware has no chance of working on anything but the most x86-like
systems.

Second software implementations SHOULD set VIRTIO_F_ACCESS_PLATFORM,
because otherwise we can't add proper handling for things like SEV or
the IBM "secure hypervisor" thing.

Last but not least a lot of wording outside the area describing these
flags really needs some major updates in terms of DMA access.