From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: Summary of LPC guest MSI discussion in Santa Fe Date: Wed, 9 Nov 2016 17:03:26 +0000 Message-ID: <20161109170326.GG17771@arm.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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <58228F71.6020108-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Don Dutile Cc: drjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, jason-NLaQJdtUoK4Be96aLqz0jA@public.gmane.org, kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, marc.zyngier-5wv7dgnIgG8@public.gmane.org, benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org, punit.agrawal-5wv7dgnIgG8@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, pranav.sawargaonkar-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, arnd-r2nGTMty4D4@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, jcm-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org, dwmw-vV1OtcyAfmbQXOPxS62xeg@public.gmane.org, Christoffer Dall , eric.auger.pro-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org List-Id: iommu@lists.linux-foundation.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