linux-kernel - Re: [RFC 0/6] Sparse HART id support

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Message-ID: <Ya4sDX974/dVEOQw@robh.at.kernel.org>
Date:   Mon, 6 Dec 2021 09:28:13 -0600
From:   Rob Herring <robh@...nel.org>
To:     Atish Patra <atishp@...shpatra.org>
Cc:     linux-kernel@...r.kernel.org, Alexandre Ghiti <alex@...ti.fr>,
        Anup Patel <anup.patel@....com>,
        Greentime Hu <greentime.hu@...ive.com>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Heinrich Schuchardt <xypron.glpk@....de>,
        Ingo Molnar <mingo@...nel.org>,
        Jisheng Zhang <jszhang@...nel.org>,
        kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org,
        linux-riscv@...ts.infradead.org, Marc Zyngier <maz@...nel.org>,
        Nanyong Sun <sunnanyong@...wei.com>,
        Nick Kossifidis <mick@....forth.gr>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Pekka Enberg <penberg@...nel.org>,
        Vincent Chen <vincent.chen@...ive.com>,
        Vitaly Wool <vitaly.wool@...sulko.com>
Subject: Re: [RFC 0/6] Sparse HART id support

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.

Rob