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:
quoted
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 :)
quoted
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