Re: [PATCH RFC 08/12] iommufd: IOCTLs for the io_pagetable

[Jason Gunthorpe via iommu <iommu@lists.linux-foundation.org>]

On Fri, Apr 29, 2022 at 04:00:14PM +1000, David Gibson wrote:
> > But I don't have a use case in mind? The simplified things I know
> > about want to attach their devices then allocate valid IOVA, they
> > don't really have a notion about what IOVA regions they are willing to
> > accept, or necessarily do hotplug.
> 
> The obvious use case is qemu (or whatever) emulating a vIOMMU.  The
> emulation code knows the IOVA windows that are expected of the vIOMMU
> (because that's a property of the emulated platform), and requests
> them of the host IOMMU.  If the host can supply that, you're good
> (this doesn't necessarily mean the host windows match exactly, just
> that the requested windows fit within the host windows).  If not,
> you report an error.  This can be done at any point when the host
> windows might change - so try to attach a device that can't support
> the requested windows, and it will fail.  Attaching a device which
> shrinks the windows, but still fits the requested windows within, and
> you're still good to go.

We were just talking about this in another area - Alex said that qemu
doesn't know the IOVA ranges? Is there some vIOMMU cases where it does?

Even if yes, qemu is able to manage this on its own - it doesn't use
the kernel IOVA allocator, so there is not a strong reason to tell the
kernel what the narrowed ranges are.

> > That is one possibility, yes. qemu seems to be using this to establish
> > a clone ioas of an existing operational one which is another usage
> > model.
> 
> Right, for qemu (or other hypervisors) the obvious choice would be to
> create a "staging" IOAS where IOVA == GPA, then COPY that into the various
> emulated bus IOASes.  For a userspace driver situation, I'm guessing
> you'd map your relevant memory pool into an IOAS, then COPY to the
> IOAS you need for whatever specific devices you're using.

qemu seems simpler, it juggled multiple containers so it literally
just copies when it instantiates a new container and does a map in
multi-container.

Jason
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