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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
