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

[Michael S. Tsirkin]

On Fri, Jul 12, 2024 at 11:06:49AM +0900, David Stevens wrote:
> 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.

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 ...

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MST