linux-kernel - Re: [PATCH v1 0/2] RISC-V: Fixes for riscv_has_extension[un]likely()'s alternative dependency

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Message-ID: <20230324113107.djp4ejnlpzn3qzrb@orel>
Date:   Fri, 24 Mar 2023 12:31:07 +0100
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Conor Dooley <conor.dooley@...rochip.com>
Cc:     palmer@...belt.com, conor@...nel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Heiko Stuebner <heiko.stuebner@...ll.eu>,
        Anup Patel <apatel@...tanamicro.com>,
        Jisheng Zhang <jszhang@...nel.org>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 0/2] RISC-V: Fixes for
 riscv_has_extension[un]likely()'s alternative dependency

On Fri, Mar 24, 2023 at 10:05:37AM +0000, Conor Dooley wrote:
> Here's my attempt at fixing both the use of an FPU on XIP kernels and
> the issue that Jason ran into where CONFIG_FPU, which needs the
> alternatives frame work for has_fpu() checks, could be enabled without
> the alternatives actually being present.
> 
> For the former, a "slow" fallback that does not use alternatives is
> added to riscv_has_extension_[un]likely() that can be used with XIP.
> Obviously, we want to make use of Jisheng's alternatives based approach
> where possible, so any users of riscv_has_extension_[un]likely() will
> want to make sure that they select RISCV_ALTERNATIVE.
> If they don't however, they'll hit the fallback path which (should,
> sparing a silly mistake from me!) behave in the same way, thus
> succeeding silently. Sounds like a
> 
> To prevent "depends on !XIP_KERNEL; select RISCV_ALTERNATIVE" spreading
> like the plague through the various places that want to check for the
> presence of extensions, and sidestep the potential silent "success"
> mentioned above, all users RISCV_ALTERNATIVE are converted from selects
> to dependencies, with the option being selected for all !XIP_KERNEL
> builds.
> 
> I know that the VDSO was a key place that Jisheng wanted to use the new
> helper rather than static branches, and I think the fallback path
> should not cause issues there.
> 
> See the thread at [1] for the prior discussion.
> 
> Cheers,
> Conor.
> 
> 1 - https://lore.kernel.org/linux-riscv/20230128172856.3814-1-jszhang@kernel.org/T/#m21390d570997145d31dd8bb95002fd61f99c6573
> 
> CC: Paul Walmsley <paul.walmsley@...ive.com>
> CC: Palmer Dabbelt <palmer@...belt.com>
> CC: Conor Dooley <conor.dooley@...rochip.com>
> CC: Heiko Stuebner <heiko.stuebner@...ll.eu>
> CC: Andrew Jones <ajones@...tanamicro.com>
> CC: Anup Patel <apatel@...tanamicro.com>
> CC: Jisheng Zhang <jszhang@...nel.org>
> CC: Andrew Jones <ajones@...tanamicro.com>
> CC: Jason A. Donenfeld <Jason@...c4.com>
> CC: linux-riscv@...ts.infradead.org
> CC: linux-kernel@...r.kernel.org
> 
> Conor Dooley (2):
>   RISC-V: add non-alternative fallback for
>     riscv_has_extension_[un]likely()
>   RISC-V: always select RISCV_ALTERNATIVE for non-xip kernels
> 
>  arch/riscv/Kconfig             | 12 ++++----
>  arch/riscv/Kconfig.erratas     |  6 ++--
>  arch/riscv/include/asm/hwcap.h | 50 ++++++++++++++++++++--------------
>  3 files changed, 38 insertions(+), 30 deletions(-)
> 
> -- 
> 2.39.2
>

LGTM, but if it was based on for-next then it could also immediately be
applied to zicboz. For the series,

Reviewed-by: Andrew Jones <ajones@...tanamicro.com>

Thanks,
drew