Re: Summary of LPC guest MSI discussion in Santa Fe

[Don Dutile]

On 11/09/2016 12:03 PM, Will Deacon wrote:
> On Tue, Nov 08, 2016 at 09:52:33PM -0500, Don Dutile wrote:
> > On 11/08/2016 06:35 PM, Alex Williamson wrote:
> > > On Tue, 8 Nov 2016 21:29:22 +0100
> > > Christoffer Dall <christoffer.dall@xxxxxxxxxx> wrote:
> > > > Is my understanding correct, that you need to tell userspace about the
> > > > location of the doorbell (in the IOVA space) in case (2), because even
> > > > though the configuration of the device is handled by the (host) kernel
> > > > through trapping of the BARs, we have to avoid the VFIO user programming
> > > > the device to create other DMA transactions to this particular address,
> > > > since that will obviously conflict and either not produce the desired
> > > > DMA transactions or result in unintended weird interrupts?
>
> Yes, that's the crux of the issue.
>
> > > Correct, if the MSI doorbell IOVA range overlaps RAM in the VM, then
> > > it's potentially a DMA target and we'll get bogus data on DMA read from
> > > the device, and lose data and potentially trigger spurious interrupts on
> > > DMA write from the device.  Thanks,
> > That's b/c the MSI doorbells are not positioned *above* the SMMU, i.e.,
> > they address match before the SMMU checks are done.  if
> > all DMA addrs had to go through SMMU first, then the DMA access could
> > be ignored/rejected.
>
> That's actually not true :( The SMMU can't generally distinguish between MSI
> writes and DMA writes, so it would just see a write transaction to the
> doorbell address, regardless of how it was generated by the endpoint.
>
> Will
So, we have real systems where MSI doorbells are placed at the same IOVA
that could have memory for a guest, but not at the same IOVA as memory on real hw ?
How are memory holes passed to SMMU so it doesn't have this issue for bare-metal
(assign an IOVA that overlaps an MSI doorbell address)?