linux-kernel - Re: [PATCH v10 3/6] riscv: mm: dma-noncoherent: nonstandard cache operations support

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Message-Id: <92c00ddb-e956-4861-af80-5f5558c9a8f5@app.fastmail.com>
Date:   Sun, 30 Jul 2023 22:35:50 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Emil Renner Berthing" <emil.renner.berthing@...onical.com>,
        "Jisheng Zhang" <jszhang@...nel.org>
Cc:     Prabhakar <prabhakar.csengg@...il.com>,
        "Conor.Dooley" <conor.dooley@...rochip.com>,
        "Geert Uytterhoeven" <geert+renesas@...der.be>,
        guoren <guoren@...nel.org>,
        "Andrew Jones" <ajones@...tanamicro.com>,
        "Paul Walmsley" <paul.walmsley@...ive.com>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        "Albert Ou" <aou@...s.berkeley.edu>,
        "Samuel Holland" <samuel@...lland.org>,
        linux-riscv@...ts.infradead.org,
        "Christoph Hellwig" <hch@...radead.org>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Krzysztof Kozlowski" <krzysztof.kozlowski+dt@...aro.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "Biju Das" <biju.das.jz@...renesas.com>,
        "Lad, Prabhakar" <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH v10 3/6] riscv: mm: dma-noncoherent: nonstandard cache operations
 support

On Sun, Jul 30, 2023, at 17:42, Emil Renner Berthing wrote:
> On Sun, 30 Jul 2023 at 17:11, Jisheng Zhang <jszhang@...nel.org> wrote:

>> > +
>> >  static inline void arch_dma_cache_wback(phys_addr_t paddr, size_t size)
>> >  {
>> >       void *vaddr = phys_to_virt(paddr);
>> >
>> > +#ifdef CONFIG_RISCV_NONSTANDARD_CACHE_OPS
>> > +     if (unlikely(noncoherent_cache_ops.wback)) {
>>
>> I'm worried about the performance impact here.
>> For unified kernel Image reason, RISCV_NONSTANDARD_CACHE_OPS will be
>> enabled by default, so standard CMO and T-HEAD's CMO platform's
>> performance will be impacted, because even an unlikely is put
>> here, the check action still needs to be done.
>
> On IRC I asked why not use a static key so the overhead is just a
> single nop when the standard CMO ops are available, but the consensus
> seemed to be that the flushing would completely dominate this branch.
> And on platforms with the standard CMO ops the branch be correctly
> predicted anyway.

Not just the flushing, but also loading back the invalidated
cache lines afterwards is just very expensive. I don't think
you would be able to measure a difference between the static
key and a correctly predicted branch on any relevant usecase here.

     Arnd