On Fri, 9 May 2025 at 09:00, Shameerali Kolothum Thodi <shameerali.kolothum.th...@huawei.com> wrote: > From: Peter Maydell <peter.mayd...@linaro.org> > > The problem here seems to me to be that in the hardware we're > > modelling the SMMU always exists, because it's in the SoC, > > but you're trying to arrange for it to be created on the > > command line, via -device. > > > > We don't have any of these problems with the current 'virt' > > board code, because we have the board code create the iommu > > (if the user asks for it via the iommu machine property), > > and it can wire it up to the PCI root complex as needed. > > Yes, currently virt creates a SMMUv3 instance and associates it with > pcie.0 by default. However, this setup isn't ideal once we introduce > support for accelerated SMMUv3, where we need to configure the > host SMMUv3 for nested stage translation. This is essential for VFIO-PCI > passthrough scenarios, where the guest manages the stage-1 page tables > and the host manages stage-2. > > In such cases, devices may be associated with different host SMMUv3s, > and having a single vSMMUv3 in the guest isn't ideal as- > > -We would need to broadcast TLBIs or perform lookups to identify the > corresponding host SMMUv3. > -The physical SMMUv3s might differ in functionality. > -Some SMMUv3 implementations offer additional virtualization features (e.g., > vCMDQ), > where the CMDQ is is directly gets mapped to the Guest and invalidations > aren't even trapped. > > Please refer to the earlier RFC [1], which proposed a new arm-smmuv3-accel > device. The main feedback on that RFC was to instead convert the existing > arm-smmuv3 into a user-creatable device, and introduce acceleration support > via an optional property: > > -device arm-smmuv3,accel=on,.. > > This series is the first step toward that goal—making the current arm-smmuv3 > device > user-creatable.
(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". -- PMM
