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Subject: Re: [virtio-dev] Re: Constraining where a guest may allocate virtio accessible resources

On Thu, Jun 18, 2020 at 04:58:40PM +0200, Jan Kiszka wrote:
> >>>>> Option 5 - Additional Device
> >>>>> ============================
> >>>>>
> >>>>> The final approach would be to tie the allocation of virtqueues to
> >>>>> memory regions as defined by additional devices. For example the
> >>>>> proposed IVSHMEMv2 spec offers the ability for the hypervisor to present
> >>>>> a fixed non-mappable region of the address space. Other proposals like
> >>>>> virtio-mem allow for hot plugging of "physical" memory into the guest
> >>>>> (conveniently treatable as separate shareable memory objects for QEMU
> >>>>> ;-).
> >>>>>
> >>>>
> >>>> I think you forgot one approach: virtual IOMMU. That is the advanced
> >>>> form of the grant table approach. The backend still "sees" the full
> >>>> address space of the frontend, but it will not be able to access all of
> >>>> it and there might even be a translation going on. Well, like IOMMUs work.
> >>>>
> >>>> However, this implies dynamics that are under guest control, namely of
> >>>> the frontend guest. And such dynamics can be counterproductive for
> >>>> certain scenarios. That's where this static windows of shared memory
> >>>> came up.
> >>>
> >>> Yes, I think IOMMU interfaces are worth investigating more too. IOMMUs
> >>> are now widely implemented in Linux and virtualization software. That
> >>> means guest modifications aren't necessary and unmodified guest
> >>> applications will run.
> >>>
> >>> Applications that need the best performance can use a static mapping
> >>> while applications that want the strongest isolation can map/unmap DMA
> >>> buffers dynamically.
> >>
> >> I do not see yet that you can model with an IOMMU a static, not guest
> >> controlled window.
> > 
> > Well basically the IOMMU will have as part of the
> > topology description and range of addresses devices behind it
> > are allowed to access. What's the problem with that?
> > 
> I didn't look at the detail of the vIOMMU from that perspective, but our
> requirement would be that it would just statically communicate to the
> guest where DMA windows are, rather than allowing the guest to configure
> that (which is the normal usage of an IOMMU).

Right, I got that - IOMMUs aren't necessarily fully configurable though.
E.g. some IOMMUs are restricted in the # of bits they can address.

> In addition, it would only address the memory transfer topic. We would
> still be left with the current issue of virtio that the hypervisor's
> device model needs to understand all supported device types.
> Jan

I'd expect the DMA API would try to paper over that likely using
bounce buffering. If you want to avoid copies, that's a harder
problem generally.


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