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

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Message-ID: <20190821014052.GA25550@infradead.org>
Date:   Tue, 20 Aug 2019 18:40:52 -0700
From:   "hch@...radead.org" <hch@...radead.org>
To:     Alan Kao <alankao@...estech.com>
Cc:     Atish Patra <Atish.Patra@....com>,
        "hch@...radead.org" <hch@...radead.org>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "anup@...infault.org" <anup@...infault.org>,
        "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 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).