> From: Jerome Glisse > Sent: Thursday, September 13, 2018 10:52 PM > [...] > AFAIK, on x86 and PPC at least, all PCIE devices are in the same group > by default at boot or at least all devices behind the same bridge. the group thing reflects physical hierarchy limitation, not changed cross boot. Please note iommu group defines the minimal isolation boundary - all devices within same group must be attached to the same iommu domain or address space, because physically IOMMU cannot differentiate DMAs out of those devices. devices behind legacy PCI-X bridge is one example. other examples include devices behind a PCIe switch port which doesn't support ACS thus cannot route p2p transaction to IOMMU. If talking about typical PCIe endpoint (with upstreaming ports all supporting ACS), you'll get one device per group. One iommu group today is attached to only one iommu domain. In the future one group may attach to multiple domains, as the aux domain concept being discussed in another thread. > > Maybe they are kernel option to avoid that and userspace init program > can definitly re-arrange that base on sysadmin policy). I don't think there is such option, as it may break isolation model enabled by IOMMU. [...] > > > That is why i am being pedantic :) on making sure there is good reasons > > > to do what you do inside VFIO. I do believe that we want a common > frame- > > > work like the one you are proposing but i do not believe it should be > > > part of VFIO given the baggages it comes with and that are not relevant > > > to the use cases for this kind of devices. > > The purpose of VFIO is clear - the kernel portal for granting generic device resource (mmio, irq, etc.) to user space. VFIO doesn't care what exactly a resource is used for (queue, cmd reg, etc.). If really pursuing VFIO path is necessary, maybe such common framework should lay down in user space, which gets all granted resource from kernel driver thru VFIO and then provides accelerator services to other processes? Thanks Kevin
