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Subject: RE: [virtio-dev] Re: [PATCH v3] Add virtio-iommu device specification

> From: Tian, Kevin
> Sent: Wednesday, May 15, 2019 1:08 PM
> To: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>; Michael S.
> Tsirkin <mst@redhat.com>
> >
> >
> > >>>> +\begin{description}
> > >>>> +  \item[VIRTIO_IOMMU_RESV_MEM_T_RESERVED (0)]
> > >>>> +    Accesses to virtual addresses in this region have undefined
> behavior.
> > >>>
> > >>> undefined?  how can platforms with such a property coexist with
> > untrusted
> > >>> devices?
> > >>
> > >> It is "undefined" because I can't enumerate all cases (don't know about
> > >> all of them). This would contain for example the RMRR regions of VT-d
> > >> (the BIOS reserving some DMA regions for itself), regions allocated for
> > >> host-managed MSIs on arm platforms, and some PCI bridge windows
> > >> (although they removed those from the Linux resv regions recently, not
> > >> certain why). Ideally we wouldn't have any, but some special cases make
> > >> it necessary.


Actually I don't think RMRR is right example here (sorry if it's my input in
our previous discussion). There was concern on RMRR providing a shared
memory region between device and platform controller, thus may cause
isolation issue:

So the basic policy for devices with RMRR is, excluding them from device
assignment, except for Intel GPU and USB. The latter two has well-understood
behavior on RMRR, thus they can be safely assigned with RMRR ignored
(i.e. RMRR region can be reused as valid IOVA region).  

As I commented below, I think it's safe to describe reserved region behavior
as either succeed with undefined behavior (e.g. in ARM MSI doorbell case, 
where a spurious interrupt on allocated doorbell for this device is triggered 
instead of desired DMA access), or cause vIOMMU unrecoverable fault due 
to invalid mappings. An untrusted device can never use reserved region to 
escape since physically IOMMU only contains verified mapping to granted
resource (MSI) or nothing.

> > >>
> > >> In general having reserved regions is incompatible with untrusted
> > >> devices, and in some cases incompatible with untrusted guests. But
> > >> choosing the policy about which devices to present to guest is up to the
> > >> platform. The host knows what the resv regions are used for, and if they
> > >> are safe enough. The guest just need to knows what to avoid.
> > >
> > > Yes but ... if you trust driver and device then what's the
> > > point of the iommu?
> >
> > Improving memory allocation. The guest can do scatter-gather for large
> > buffers, instead of allocating big contiguous chunks of guest-physical
> > addresses. And with device pass-through, any memory used for DMA has to
> > be pinned (since devices generally don't support page faults). So when
> > assigning an endpoint to a guest, without a vIOMMU all of the guest
> > memory has to be pinned upfront. With an IOMMU only the memory that
> will
> > actually be used for DMA is pinned.
> I'd not vote using current vIOMMU just for avoiding pin instead of for
> isolation purpose... the map/unmap overhead is high except you only
> talking about static mapping e.g. in guest user space driver (e.g. DPDK)
> (but then it's actually for isolation purpose between user and kernel).
>  btw we are working on a lighter-weight approach (aim <5% perf drop
> for most cases) while allowing host to fully introspect guest DMA activities.
> Likely we'll introduce a new virtio-iommu capability for this purpose,
> instead of using current MAP/UNMAP primitives. Stay tuned and we'll
> send out RFC soon once getting good result. :-)
> And I don't think having reserved regions is incompatible with untrusted
> devices. Favoring reserved region is a functional requirement so guest
> driver can operate assigned device in a good shape. Violating reserved
> region requirement (e.g. map guest memory page to reserved region) just
> means device access to this page doesn't provide desired behavior, since
> host will always guarantee safe behavior in physical IOMMU on those
> regions:
> - it is either mapped to fixed resource (e.g ARM MSI doorbell)
> which is associated with this device and thus safe
> - or map to nothing
> I don't think architecturally we break any isolation implication on
> IOMMU here.
> Thanks
> Kevin

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