On Fri, Jul 12, 2024 at 11:06:49AM +0900, David Stevens wrote: > On Thu, Jul 11, 2024 at 7:56 PM Alyssa Ross <h...@alyssa.is> wrote: > > > > Adding David Stevens, who implemented SHMEM_MAP and SHMEM_UNMAP in > > crosvm a couple of years ago. > > > > David, I'd be particularly interested for your thoughts on the MEM_READ > > and MEM_WRITE commands, since as far as I know crosvm doesn't implement > > anything like that. The discussion leading to those being added starts > > here: > > > > https://lore.kernel.org/qemu-devel/20240604185416.gb90...@fedora.redhat.com/ > > > > It would be great if this could be standardised between QEMU and crosvm > > (and therefore have a clearer path toward being implemented in other VMMs)! > > Setting aside vhost-user for a moment, the DAX example given by Stefan > won't work in crosvm today. > > Is universal access to virtio shared memory regions actually mandated > by the virtio spec? Copying from virtiofs DAX to virtiofs sharing > seems reasonable enough, but what about virtio-pmem to virtio-blk? > What about screenshotting a framebuffer in virtio-gpu shared memory to > virtio-scsi? I guess with some plumbing in the VMM, it's solvable in a > virtualized environment. But what about when you have real hardware > that speaks virtio involved? That's outside my wheelhouse, but it > doesn't seem like that would be easy to solve.
Yes, it can work for physical devices if allowed by host configuration. E.g. VFIO supports that I think. Don't think VDPA does. > For what it's worth, my interpretation of the target scenario: > > > Other backends don't see these mappings. If the guest submits a vring > > descriptor referencing a mapping to another backend, then that backend > > won't be able to access this memory > > is that it's omitting how the implementation is reconciled with > section 2.10.1 of v1.3 of the virtio spec, which states that: > > > References into shared memory regions are represented as offsets from > > the beginning of the region instead of absolute memory addresses. Offsets > > are used both for references between structures stored within shared > > memory and for requests placed in virtqueues that refer to shared memory. > > My interpretation of that statement is that putting raw guest physical > addresses corresponding to virtio shared memory regions into a vring > is a driver spec violation. > > -David This really applies within device I think. Should be clarified ... -- MST
