Summary of LPC guest MSI discussion in Santa Fe

[Alex Williamson]

On Wed, 9 Nov 2016 22:25:22 +0000
Will Deacon <will.deacon at arm.com> wrote:

> On Wed, Nov 09, 2016 at 03:17:09PM -0700, Alex Williamson wrote:
> > On Wed, 9 Nov 2016 20:31:45 +0000
> > Will Deacon <will.deacon at arm.com> wrote:  
> > > On Wed, Nov 09, 2016 at 08:23:03PM +0100, Christoffer Dall wrote:  
> > > > 
> > > > (I suppose it's technically possible to get around this issue by letting
> > > > QEMU place RAM wherever it wants but tell the guest to never use a
> > > > particular subset of its RAM for DMA, because that would conflict with
> > > > the doorbell IOVA or be seen as p2p transactions.  But I think we all
> > > > probably agree that it's a disgusting idea.)    
> > > 
> > > Disgusting, yes, but Ben's idea of hotplugging on the host controller with
> > > firmware tables describing the reserved regions is something that we could
> > > do in the distant future. In the meantime, I don't think that VFIO should
> > > explicitly reject overlapping mappings if userspace asks for them.  
> > 
> > I'm confused by the last sentence here, rejecting user mappings that
> > overlap reserved ranges, such as MSI doorbell pages, is exactly how
> > we'd reject hot-adding a device when we meet such a conflict.  If we
> > don't reject such a mapping, we're knowingly creating a situation that
> > potentially leads to data loss.  Minimally, QEMU would need to know
> > about the reserved region, map around it through VFIO, and take
> > responsibility (somehow) for making sure that region is never used for
> > DMA.  Thanks,  
> 
> Yes, but my point is that it should be up to QEMU to abort the hotplug, not
> the host kernel, since there may be ways in which a guest can tolerate the
> overlapping region (e.g. by avoiding that range of memory for DMA).

The VFIO_IOMMU_MAP_DMA ioctl is a contract, the user ask to map a range
of IOVAs to a range of virtual addresses for a given device.  If VFIO
cannot reasonably fulfill that contract, it must fail.  It's up to QEMU
how to manage the hotplug and what memory regions it asks VFIO to map
for a device, but VFIO must reject mappings that it (or the SMMU by
virtue of using the IOMMU API) know to overlap reserved ranges.  So I
still disagree with the referenced statement.  Thanks,

Alex