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

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Message-ID: <20190821012921.GA30187@andestech.com>
Date:   Wed, 21 Aug 2019 09:29:22 +0800
From:   Alan Kao <alankao@...estech.com>
To:     Atish Patra <Atish.Patra@....com>
CC:     "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 Tue, Aug 20, 2019 at 08:28:36PM +0000, Atish Patra wrote:
> On Tue, 2019-08-20 at 02:22 -0700, hch@...radead.org wrote:
> > On Tue, Aug 20, 2019 at 08:42:19AM +0000, Atish Patra wrote:
> > > cmask NULL is pretty common case and we would  be unnecessarily
> > > executing bunch of instructions everytime while not saving much.
> > > Kernel
> > > still have to make an SBI call and OpenSBI is doing a local flush
> > > anyways.
> > > 
> > > Looking at the code again, I think we can just use cpumask_weight
> > > and
> > > do local tlb flush only if local cpu is the only cpu present. 
> > > 
> > > Otherwise, it will just fall through and call
> > > sbi_remote_sfence_vma().
> > 
> > Maybe it is just time to split the different cases at a higher level.
> > The idea to multiple everything onto a single function always seemed
> > odd to me.
> > 
> > FYI, here is what I do for the IPI based tlbflush for the native S-
> > mode
> > clint prototype, which seems much easier to understand:
> > 
> > http://git.infradead.org/users/hch/riscv.git/commitdiff/ea4067ae61e20fcfcf46a6f6bd1cc25710ce3afe
> 
> This does seem a lot cleaner to me. We can reuse some of the code for
> this patch as well. Based on NATIVE_CLINT configuration, it will send
> an IPI or SBI call.

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.

> 
> I can rebase my patch on top of yours and I can send it together or you
> can include in your series.
> 
> Let me know your preference.
> 
> -- 
> Regards,
> Atish

Best,
Alan