Subject: Summary of LPC guest MSI discussion in Santa Fe

From: [email-masked] (Will Deacon)
Date: Wed, 9 Nov 2016 17:03:26 +0000

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 <[email-masked]> 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