On Wed, Apr 27, 2016 at 7:54 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > On Wed, Apr 27, 2016 at 07:43:07AM -0700, Andy Lutomirski wrote: >> On Wed, Apr 27, 2016 at 7:38 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: >> > On Wed, Apr 27, 2016 at 07:31:43AM -0700, Andy Lutomirski wrote: >> >> On Wed, Apr 27, 2016 at 7:23 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote: >> >> > On Wed, Apr 27, 2016 at 04:37:04PM +0300, Michael S. Tsirkin wrote: >> >> >> One correction: it's a feature of the device in the system. >> >> >> There could be a mix of devices bypassing and not >> >> >> bypassing the IOMMU. >> >> > >> >> > No, it really is not. A device can't chose to bypass the IOMMU. But the >> >> > IOMMU can chose to let the device bypass. So any fix here belongs >> >> > into the platform/iommu code too and not into some driver. >> >> > >> >> >> Sounds good. And a way to detect appropriate devices could >> >> >> be by looking at the feature flag, perhaps? >> >> > >> >> > Again, no! The way to detect that is to look into the iommu description >> >> > structures provided by the firmware. They provide everything necessary >> >> > to tell the iommu code which devices are not translated. >> >> > >> >> >> >> Except on PPC and SPARC. As far as I know, those are the only >> >> problematic platforms. >> >> >> >> Is it too late to *disable* QEMU's q35-iommu thingy until it can be >> >> fixed to report correct data in the DMAR tables? >> >> >> >> --Andy >> > >> > Meaning virtio or assigned devices? >> > For virtio - it's way too late since these are working configurations. >> > For assigned devices - they don't work on x86 so it doesn't have >> > to be disabled, it's safe to ignore. >> >> I mean actually prevent QEMU from running in q35-iommu mode with any >> virtio devices attached or maybe even turn off q35-iommu mode entirely >> [1]. Doesn't it require that the user literally pass the word >> "experimental" into QEMU right now? It did at some point IIRC. >> >> The reason I'm asking is that, other than q35-iommu, QEMU's virtio >> devices *don't* bypass the IOMMU except on PPC and SPARC, simply >> because there is no other configuration AFAICT that has virtio and and >> IOMMU. So maybe the right solution is to fix q35-iommu to use DMAR >> correctly (thus breaking q35-iommu users with older guest kernels, >> which hopefully don't actually exist) and to come up with a PPC- and >> SPARC-specific solution, or maybe OpenFirmware-specific solution, to >> handle PPC and SPARC down the road. >> >> [1] I'm pretty sure I emailed the QEMU list before q35-iommu ever >> showed up in a release asking the QEMU team to please not do that >> until this issue was resolved. Sadly, that email was ignored :( >> >> --Andy > > Sorry, I didn't make myself clear. > Point is, QEMU is not the only virtio implementation out there. > So we can't know no virtio implementations have an IOMMU as long as > linux supports this IOMMU. > virtio always used physical addresses since it was born and if it > changes that it must do this in a way that does not break existing > users. Is there any non-QEMU virtio implementation can provide an IOMMU-bypassing virtio device on a platform that has a nontrivial IOMMU? --Andy -- 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
