Subject: Summary of LPC guest MSI discussion in Santa Fe

From: [email-masked] (Don Dutile)
Date: Wed, 9 Nov 2016 13:59:07 -0500

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 <[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
>
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)?