On Fri, 9 May 2025 at 11:46, Daniel P. Berrangé <berra...@redhat.com> wrote: > > On Fri, May 09, 2025 at 11:37:14AM +0100, Peter Maydell wrote: > > (I want to start here by saying that I appreciate that I'm > > coming in without having read the previous discussion, so > > this is kind of going back over ground you've already > > been through.) > > > > I agree that rather than having an entirely separate "SMMU with > > acceleration" it would be better to have it be a property on > > the SMMU device. But why do we need it to be user created? > > Making it user-created leads into all kinds of tricky areas > > mostly surrounding the fact that QEMU right now simply doesn't > > support having user-created sysbus devices and other kinds > > of device with complex wiring-up. -device is really intended > > for "this is a model of a device that in real hardware is > > pluggable and has basically one connection, like a PCI card > > has a PCI-slot". > > In terms of "why does it need to be user created" - the goal was to expose > multiple SMMUs to the guest, each associated with a separate physical SMMU. > IIUC, each physical NUMA node would have its own SMMU. > > So configuring a guest VM will require creating multiple PXBs, one for > each virtual NUMA node, and then creating SMMUs for each PXB. > > Since there was a need for the user to create SMMUs for the PXBs, the > question was then raised, why shouldn't the default SMMU also be > user creatable in the same way, so that mgmt apps like libvirt have > a single way to configure the SMMUs with -device.
Sure, the default "there's just one pci bridge and either no SMMU or one SMMU" isn't that special. But we don't have good infrastructure for creating sysbus devices on the command line, whether it's the default SMMU or the extra SMMUs or a UART or anything else. I guess the dynamic_sysbus stuff works, but I've never really liked it (it's basically "the board will magically do the right thing", and to some extent it's working around the way we have very patchy support for "I want to configure a board the device created rather than configuring a device I am creating"). -- PMM
