Re: [RFC PATCH v2 0/5] vhost-user: Add SHMEM_MAP/UNMAP requests

[David Stevens]

On Thu, Jul 11, 2024 at 7:56 PM Alyssa Ross <hi@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.GB90471@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.

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