Summary of LPC guest MSI discussion in Santa Fe

[Alex Williamson]

On Fri, 11 Nov 2016 12:19:44 +0100
Joerg Roedel <joro at 8bytes.org> wrote:

> On Thu, Nov 10, 2016 at 10:46:01AM -0700, Alex Williamson wrote:
> > In the case of x86, we know that DMA mappings overlapping the MSI
> > doorbells won't be translated correctly, it's not a valid mapping for
> > that range, and therefore the iommu driver backing the IOMMU API
> > should describe that reserved range and reject mappings to it.  
> 
> The drivers actually allow mappings to the MSI region via the IOMMU-API,
> and I think it should stay this way also for other reserved ranges.
> Address space management is done by the IOMMU-API user already (and has
> to be done there nowadays), be it a DMA-API implementation which just
> reserves these regions in its address space allocator or be it VFIO with
> QEMU, which don't map RAM there anyway. So there is no point of checking
> this again in the IOMMU drivers and we can keep that out of the
> mapping/unmapping fast-path.

It's really just a happenstance that we don't map RAM over the x86 MSI
range though.  That property really can't be guaranteed once we mix
architectures, such as running an aarch64 VM on x86 host via TCG.
AIUI, the MSI range is actually handled differently than other DMA
ranges, so a iommu_map() overlapping a range that the iommu cannot map
should fail just like an attempt to map beyond the address width of the
iommu.
 
> > For PCI devices userspace can examine the topology of the iommu group
> > and exclude MMIO ranges of peer devices based on the BARs, which are
> > exposed in various places, pci-sysfs as well as /proc/iomem.  For
> > non-PCI or MSI controllers... ???  
> 
> Right, the hardware resources can be examined. But maybe this can be
> extended to also cover RMRR ranges? Then we would be able to assign
> devices with RMRR mappings to guests.

RMRRs are special in a different way, the VT-d spec requires that the
OS honor RMRRs, the user has no responsibility (and currently no
visibility) to make that same arrangement.  In order to potentially
protect the physical host platform, the iommu drivers should prevent a
user from remapping RMRRS.  Maybe there needs to be a different
interface used by untrusted users vs in-kernel drivers, but I think the
kernel really needs to be defensive in the case of user mappings, which
is where the IOMMU API is rooted.  Thanks,

Alex