[RFC 0/6] Sparse HART id support

Rob Herring robh at kernel.org
Mon Dec 6 07:28:13 PST 2021

On Fri, Dec 03, 2021 at 04:20:32PM -0800, Atish Patra wrote:
> Currently, sparse hartid is not supported for Linux RISC-V for the following
> reasons.
> 1. Both spinwait and ordered booting method uses __cpu_up_stack/task_pointer
>    which is an array size of NR_CPUs.
> 2. During early booting, any hartid greater than NR_CPUs are not booted at all.
> 3. riscv_cpuid_to_hartid_mask uses struct cpumask for generating hartid bitmap.
> 4. SBI v0.2 implementation uses NR_CPUs as the maximum hartid number while
>    generating hartmask.
> In order to support sparse hartid, the hartid & NR_CPUS needs to be disassociated
> which was logically incorrect anyways. NR_CPUs represent the maximum logical|
> CPU id configured in the kernel while the hartid represent the physical hartid
> stored in mhartid CSR defined by the privilege specification. Thus, hartid
> can have much greater value than logical cpuid.

We already have a couple of architectures with logical to physical CPU 
id maps. See cpu_logical_map. Can we make that common and use it here? 
That would also possibly allow for common populating the map from DT.


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