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

From: Benjamin Herrenschmidt
Date: Sun Aug 05 2018 - 17:27:23 EST

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:
> > - 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 ?