Re: [RFC 0/4] Virtio uses DMA API for all devices

On Sun, 2018-08-05 at 00:29 -0700, Christoph Hellwig wrote:

On Sun, Aug 05, 2018 at 11:10:15AM +1000, Benjamin Herrenschmidt wrote:

quoted

 - One you have rejected, which is to have a way for "no-iommu" virtio
(which still doesn't use an iommu on the qemu side and doesn't need
to), to be forced to use some custom DMA ops on the VM side.

 - One, which sadly has more overhead and will require modifying more
pieces of the puzzle, which is to make qemu uses an emulated iommu.
Once we make qemu do that, we can then layer swiotlb on top of the
emulated iommu on the guest side, and pass that as dma_ops to virtio.

Or number three:  have a a virtio feature bit that tells the VM
to use whatever dma ops the platform thinks are appropinquate for
the bus it pretends to be on.  Then set a dma-range that is limited
to your secure memory range (if you really need it to be runtime
enabled only after a device reset that rescans) and use the normal
dma mapping code to bounce buffer.

Who would set this bit ? qemu ? Under what circumstances ?

What would be the effect of this bit while VIRTIO_F_IOMMU is NOT set,
ie, what would qemu do and what would Linux do ? I'm not sure I fully
understand your idea.

I'm trying to understand because the limitation is not a device side
limitation, it's not a qemu limitation, it's actually more of a VM
limitation. It has most of its memory pages made inaccessible for
security reasons. The platform from a qemu/KVM perspective is almost
entirely normal.

So I don't understand when would qemu set this bit, or should it be set
by the VM at runtime ?

Cheers,
Ben.