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

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Date:   Mon, 13 Dec 2021 17:11:21 -0600
From:   Rob Herring <robh@...nel.org>
To:     Atish Patra <atishp@...shpatra.org>
Cc:     "linux-kernel@...r.kernel.org List" <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 General <kvm@...r.kernel.org>,
        linux-riscv <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 Mon, Dec 13, 2021 at 3:27 PM Atish Patra <atishp@...shpatra.org> wrote:
>
> On Mon, Dec 6, 2021 at 7:28 AM Rob Herring <robh@...nel.org> wrote:
> >
> > 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?
>
> Yes. We can move the cpu_logical_map(which is a macro) &
> __cpu_logical_map(actual array with NR_CPUS size)
> to common code so that all the architecture can use it instead of
> defining it separately.

IIRC, the macro is what varies by arch and I would move to static
inlines rather than supporting:

cpu_logical_map(cpu) = 0xdeadbeef;

>
> > That would also possibly allow for common populating the map from DT.
> >
>
> I didn't understand this part. The mapping is populated at run time
> [1] as the boot cpu can be any hart in RISC-V.
> That booting hart will be mapped to cpu 0. All others will be mapped
> based on how the cpu node is laid out in the DT.
> Do you mean we can move the 2nd part to common code as well ?

Yes, as the DT platforms just loop thru the cpu nodes and fill the
logical map based on 'reg', I don't think that needs to be per arch
once we have a common map. But not asking for that now.

Rob