From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Wed, 9 Nov 2016 17:03:26 +0000 Subject: Summary of LPC guest MSI discussion in Santa Fe In-Reply-To: <58228F71.6020108@redhat.com> References: <1478209178-3009-1-git-send-email-eric.auger@redhat.com> <20161103220205.37715b49@t450s.home> <20161108024559.GA20591@arm.com> <20161108202922.GC15676@cbox> <20161108163508.1bcae0c2@t450s.home> <58228F71.6020108@redhat.com> Message-ID: <20161109170326.GG17771@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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 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