linux-kernel - Re: [v2 PATCH] RISC-V: Optimize tlb flush path.

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Date:   Wed, 21 Aug 2019 09:22:48 +0530
From:   Anup Patel <anup@...infault.org>
To:     "hch@...radead.org" <hch@...radead.org>
Cc:     Alan Kao <alankao@...estech.com>,
        Atish Patra <Atish.Patra@....com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "palmer@...ive.com" <palmer@...ive.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "schwab@...ux-m68k.org" <schwab@...ux-m68k.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "allison@...utok.net" <allison@...utok.net>
Subject: Re: [v2 PATCH] RISC-V: Optimize tlb flush path.

On Wed, Aug 21, 2019 at 7:10 AM hch@...radead.org <hch@...radead.org> wrote:
>
> On Wed, Aug 21, 2019 at 09:29:22AM +0800, Alan Kao wrote:
> > IMHO, this approach should be avoided because CLINT is compatible to but
> >  not mandatory in the privileged spec.  In other words, it is possible that
> > a Linux-capable RISC-V platform does not contain a CLINT component but
> > rely on some other mechanism to deal with SW/timer interrupts.
>
> Hi Alan,
>
> at this point the above is just a prototype showing the performance
> improvement if we can inject IPIs and timer interrups directly from
> S-mode and delivered directly to S-mode.  It is based on a copy of
> the clint IPI block currently used by SiFive, qemu, Ariane and Kendryte.
>
> If the experiment works out (which I think it does), I'd like to
> define interfaces for the unix platform spec to make something like
> this available.  My current plan for that is to have one DT node
> each for the IPI registers, timer cmp and time val register each
> as MMIO regions.  This would fit the current clint block but also
> allow other register layouts.  Is that something you'd be fine with?
> If not do you have another proposal?  (note that eventually the
> dicussion should move to the unix platform spec list, but now that
> I have you here we can at least brain storm a bit).

I agree that IPI mechanism should be standardized for RISC-V but I
don't support the idea of mandating CLINT as part of the UNIX
platform spec. For example, the AndesTech SOC does not use CLINT
instead they have PLMT for per-HART timer and PLICSW for per-HART
IPIs.

IMHO, we can also think of:
RISC-V Timer Extension - For per-HART timer access to M-mode
and S-mode
RISC-V IPI Extension - HART IPI injection

Regards,
Anup