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

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Message-ID: <20230324100538.3514663-1-conor.dooley@microchip.com>
Date:   Fri, 24 Mar 2023 10:05:37 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     <palmer@...belt.com>
CC:     <conor@...nel.org>, <conor.dooley@...rochip.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Heiko Stuebner <heiko.stuebner@...ll.eu>,
        "Andrew Jones" <ajones@...tanamicro.com>,
        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: [PATCH v1 0/2] RISC-V: Fixes for riscv_has_extension[un]likely()'s alternative dependency

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