linux-kernel - Re: [PATCH v6 0/6] RISC-V Hardware Probing User Interface

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Message-ID: <20230411-primate-rice-a5c102f90c6c@wendy>
Date:   Tue, 11 Apr 2023 15:16:58 +0100
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Evan Green <evan@...osinc.com>
CC:     Palmer Dabbelt <palmer@...osinc.com>, <slewis@...osinc.com>,
        <heiko@...ech.de>, Conor Dooley <conor@...nel.org>,
        <vineetg@...osinc.com>, Albert Ou <aou@...s.berkeley.edu>,
        Andrew Bresticker <abrestic@...osinc.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anup Patel <apatel@...tanamicro.com>,
        Atish Patra <atishp@...osinc.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Celeste Liu <coelacanthus@...look.com>,
        Guo Ren <guoren@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Heiko Stuebner <heiko.stuebner@...ll.eu>,
        Jann Horn <jannh@...gle.com>,
        Jisheng Zhang <jszhang@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Ley Foon Tan <leyfoon.tan@...rfivetech.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Mark Brown <broonie@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Philipp Tomsich <philipp.tomsich@...ll.eu>,
        Samuel Holland <samuel@...lland.org>,
        Shuah Khan <shuah@...nel.org>,
        Sunil V L <sunilvl@...tanamicro.com>,
        Tobias Klauser <tklauser@...tanz.ch>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>,
        <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v6 0/6] RISC-V Hardware Probing User Interface

Hey Evan,

On Fri, Apr 07, 2023 at 04:10:57PM -0700, Evan Green wrote:
> 
> There's been a bunch of off-list discussions about this, including at
> Plumbers.  The original plan was to do something involving providing an
> ISA string to userspace, but ISA strings just aren't sufficient for a
> stable ABI any more: in order to parse an ISA string users need the
> version of the specifications that the string is written to, the version
> of each extension (sometimes at a finer granularity than the RISC-V
> releases/versions encode), and the expected use case for the ISA string
> (ie, is it a U-mode or M-mode string).  That's a lot of complexity to
> try and keep ABI compatible and it's probably going to continue to grow,
> as even if there's no more complexity in the specifications we'll have
> to deal with the various ISA string parsing oddities that end up all
> over userspace.
> 
> Instead this patch set takes a very different approach and provides a set
> of key/value pairs that encode various bits about the system.  The big
> advantage here is that we can clearly define what these mean so we can
> ensure ABI stability, but it also allows us to encode information that's
> unlikely to ever appear in an ISA string (see the misaligned access
> performance, for example).  The resulting interface looks a lot like
> what arm64 and x86 do, and will hopefully fit well into something like
> ACPI in the future.
> 
> The actual user interface is a syscall, with a vDSO function in front of
> it. The vDSO function can answer some queries without a syscall at all,
> and falls back to the syscall for cases it doesn't have answers to.
> Currently we prepopulate it with an array of answers for all keys and
> a CPU set of "all CPUs". This can be adjusted as necessary to provide
> fast answers to the most common queries.
> 
> An example series in glibc exposing this syscall and using it in an
> ifunc selector for memcpy can be found at [1].
> 
> I was asked about the performance delta between this and something like
> sysfs. I created a small test program [2] and ran it on a Nezha D1
> Allwinner board. Doing each operation 100000 times and dividing, these
> operations take the following amount of time:
>  - open()+read()+close() of /sys/kernel/cpu_byteorder: 3.8us
>  - access("/sys/kernel/cpu_byteorder", R_OK): 1.3us
>  - riscv_hwprobe() vDSO and syscall: .0094us
>  - riscv_hwprobe() vDSO with no syscall: 0.0091us
> 
> These numbers get farther apart if we query multiple keys, as sysfs will
> scale linearly with the number of keys, where the dedicated syscall
> stays the same. To frame these numbers, I also did a tight
> fork/exec/wait loop, which I measured as 4.8ms. So doing 4
> open/read/close operations is a delta of about 0.3%, versus a single vDSO
> call is a delta of essentially zero.

Two nits w.r.t. build bot complaints...

On patch 2:
arch/riscv/include/uapi/asm/unistd.h:54:1: warning: initializer overrides prior initialization of this subobject [-Winitializer-overrides]
I think this one is kinda spurious, all of the syscalls complain like
this (and do on arm64 too IIRC). There was a patch from Guo somewhere to
disable -Winitializer-overrides in this case, I should go find out what
happened to it.

On patch 4:
arch/riscv/kernel/cpufeature.c:29:1: warning: symbol '__pcpu_scope_misaligned_access_speed' was not declared. Should it be static?

Probably because cos cpufeature.c doesn't include the header of the same
name... Perhaps Palmer could fix that one up on application?

Cheers,
Conor.


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