linux-kernel - Re: [PATCH v7 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 4 Apr 2023 07:59:49 +0100
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Christoph Hellwig <hch@...radead.org>,
        Prabhakar <prabhakar.csengg@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Heiko Stübner <heiko@...ech.de>,
        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>,
        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 v7 1/6] riscv: mm: dma-noncoherent: Switch using function
 pointers for cache management

On Tue, Apr 04, 2023 at 08:50:16AM +0200, Arnd Bergmann wrote:
> On Tue, Apr 4, 2023, at 07:29, Christoph Hellwig wrote:
> > On Thu, Mar 30, 2023 at 09:42:12PM +0100, Prabhakar wrote:
> >> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> >> 
> >> Currently, selecting which CMOs to use on a given platform is done using
> >> and ALTERNATIVE_X() macro. This was manageable when there were just two
> >> CMO implementations, but now that there are more and more platforms coming
> >> needing custom CMOs, the use of the ALTERNATIVE_X() macro is unmanageable.
> >> 
> >> To avoid such issues this patch switches to use of function pointers
> >> instead of ALTERNATIVE_X() macro for cache management (the only drawback
> >> being performance over the previous approach).
> >> 
> >> void (*clean_range)(unsigned long addr, unsigned long size);
> >> void (*inv_range)(unsigned long addr, unsigned long size);
> >> void (*flush_range)(unsigned long addr, unsigned long size);
> >> 
> >
> > NAK.  Function pointers for somthing high performance as cache
> > maintainance is a complete no-go.
> 
> As we already discussed, this is now planned to use a direct
> branch to the zicbom version when the function pointer is NULL,
> which should be a fast predicted branch on all standard implementations
> and only be slow on the nonstandard ones, which I think is a reasonable
> compromise.
> 
> I'm also not sure I'd actually consider this a fast path, since
> there is already a significant cost in accessing the invalidated
> cache lines afterwards, which is likely going to be much higher than
> the cost of an indirect branch.
> 
> I suppose an alternative would be to use the linux/static_call.h
> infrastructure to avoid the overhead of indirect branches altogether.
> Right now, this has no riscv specific implementation though, so
> using it just turns into a regular indirect branch until someone
> implements static_call.

Someone actually posted an implementation for that the other day:
https://patchwork.kernel.org/project/linux-riscv/patch/tencent_A8A256967B654625AEE1DB222514B0613B07@qq.com/

I haven't looked at it at all, other than noticing the build issues
outside of for !rv64 || !mmu, so I have no idea what state it is
actually in.


Download attachment "signature.asc" of type "application/pgp-signature" (229 bytes)