On Wed, Feb 28, 2024 at 6:21 AM Peter Maydell <peter.mayd...@linaro.org> wrote: > > On Tue, 27 Feb 2024 at 22:24, Joe Komlodi <koml...@google.com> wrote: > > This adds requester IDs to ARM CPUs and adds a "user-defined" memory > > attribute. > > > > The requester ID on ARM CPUs is there because I've seen some cases where > > there's an IOMMU between a CPU and memory that uses the CPU's requester > > ID to look up how it should translate, such as an SMMU TBU or some other > > IOMMU-like device. > > For a specific downstream example I've seen, Xilinx overrides CPU > > attributes with ones passed in by an object property in order to have > > their IOMMUs work: > > https://github.com/Xilinx/qemu/blob/23b643ba1683a47ef49447a45643fe2172d6f8ca/accel/tcg/cputlb.c#L1127. > > The object property with the memory attributes is declared here, for > > reference: > > https://github.com/Xilinx/qemu/blob/23b643ba1683a47ef49447a45643fe2172d6f8ca/target/arm/cpu.c#L1310. > > > > The user-defined attribute represents optional user signals that are a > > part of AMBA-AXI. As the name suggests, these are defined > > per-implementation and devices that receive these have their own > > interpretation of what the user-defined attribute means. > > > > We add them in CPUs and PCI transactions, because some of their > > attributes are set in functions in ways that are not user-facing. DMAs > > or other devices that set attributes (using address_space_rw or some > > other means), can add them on a per-device basis. > > So as far as I can see, this patchset defines a bunch of mechanism, > but no actual users: no device looks at these new memattrs, no board > code sets the properties. I don't really want to add this without > an upstream usecase for it.
Yeah, I believe the current use-cases for this series are mostly downstream. It's possible that there's an upstream device that might benefit from it, but I'm not aware of one. Is the concern the usefulness of the series, or the worry about it bit-rotting? If it's the latter, would a qtest be alright to make sure it doesn't rot? Thanks, Joe > > thanks > -- PMM
