Hey Michael, Thanks a bunch for having a detailed and quick look!
On 10.08.26 08:23, Michael S. Tsirkin wrote: > On Sun, Aug 09, 2026 at 06:19:58PM +0000, Alexander Graf wrote: >> Virtio drivers use guest memory to back virtqueues and their buffers. >> That means a VMM needs to be able to map guest memory. That is ok in the >> normal virt case. It gets icky with confidential computing (where we use >> swiotlb as workaround) and it defeats the purpose of isolated vhost-user >> backing devices, because they end up with full RAM access to the guest. >> >> So instead, I'm proposing an extension to virtio which allows it to give >> each virtio device its own dedicated memory region to communicate with the >> host, called DMB (Device Memory Buffer). A trusted hypervisor can force >> DMB to be present, which then enables safer, more isolated and resilient >> communication between guest and host. >> >> With DMB, the device provides a shared memory region that both parties >> agree is the full memory map both have access to. All memory offsets >> that previously would have been into guest RAM, are then offsets into >> this shared memory buffer region. One nice property of this is that it >> is a generic mechanism in the virtio transport layer, so higher level >> drivers work unmodified. >> >> I was exploring to use swiotlb instead to create individual pools. But >> that approach has multiple downsides: >> >> 1. Swiotlb is an OS primitive which is not available in all Operating >> Systems. DMB however lives in the virtio transport layer, which means we >> can add support for it in any OS independent of generic layers. This >> helps with Windows support. > > How does it help, if you are going to put a pool in > the driver, put a pool in the driver. Maybe with virtio mem to > simplify allocation. I'm not sure I understand the suggestion :) > >> 2. We munge DMA space together. DMB provides a separate DMA space per >> virtio device. This means we can for example implement a device in >> vhost-user and give the implementing process only visibility to the DMB >> region, not all of guest memory. That reduces the exposure the >> vhost-user provider has, improving security. >> >> 3. Devices can opt-in. A hypervisor can choose to use standard virtio >> semantics for self-implemented devices (e.g. NSM), while requiring DMB >> for devices implemented by less trustworthy providers. The >> non-trustworthy devices do not get any visibility into the trustworthy >> ones, even with DMB in place for both. > > So I am not sure whether the implication is that it's purely a software > construct. But if it is, can we extend virtio iommu to > add a way to discover and enforce trust boundaries? > And maybe translate offsets to BARs, if that is desired? > > It seems to be that the result would be that we don't need fiddly > special casing in virtio ring specifically, and a lot of things like > pre-mapped dma will begin to work. On thing I'm trying to avoid is dynamicity. Anything that dynamically changes visibility or needs state tracking is something that can go wrong. By keeping everything self-contained within the guest, I can reason about what is visible and what is not easily. Especially for confidential computing, IMHO static wins over dynamic in general. Or did I misunderstand your suggestion? As to pure software construct: With CXL, you can implement the exact same protocol on real hardware as well. All it takes is cache coherency of the BAR (or whatever the transport uses) region. Alex
