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Date:   Fri, 24 Mar 2023 11:37:05 +0000
From:   Conor Dooley <conor.dooley@microchip.com>
To:     Andrew Jones <ajones@ventanamicro.com>
CC:     <palmer@dabbelt.com>, <conor@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Heiko Stuebner <heiko.stuebner@vrull.eu>,
        Anup Patel <apatel@ventanamicro.com>,
        Jisheng Zhang <jszhang@kernel.org>,
        "Jason A . Donenfeld" <Jason@zx2c4.com>,
        <linux-riscv@lists.infradead.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v1 0/2] RISC-V: Fixes for
 riscv_has_extension[un]likely()'s alternative dependency
Message-ID: <e7a93223-54e4-4c97-b121-5d1be3af0d65@spud>
References: <20230324100538.3514663-1-conor.dooley@microchip.com>
 <20230324113107.djp4ejnlpzn3qzrb@orel>
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On Fri, Mar 24, 2023 at 12:31:07PM +0100, Andrew Jones wrote:
> 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.
> >=20
> > 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
> >=20
> > 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.
> >=20
> > 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.
> >=20
> > See the thread at [1] for the prior discussion.
> >=20
> > Cheers,
> > Conor.
> >=20
> > 1 - https://lore.kernel.org/linux-riscv/20230128172856.3814-1-jszhang@k=
ernel.org/T/#m21390d570997145d31dd8bb95002fd61f99c6573
> >=20
> > CC: Paul Walmsley <paul.walmsley@sifive.com>
> > CC: Palmer Dabbelt <palmer@dabbelt.com>
> > CC: Conor Dooley <conor.dooley@microchip.com>
> > CC: Heiko Stuebner <heiko.stuebner@vrull.eu>
> > CC: Andrew Jones <ajones@ventanamicro.com>
> > CC: Anup Patel <apatel@ventanamicro.com>
> > CC: Jisheng Zhang <jszhang@kernel.org>
> > CC: Andrew Jones <ajones@ventanamicro.com>
> > CC: Jason A. Donenfeld <Jason@zx2c4.com>
> > CC: linux-riscv@lists.infradead.org
> > CC: linux-kernel@vger.kernel.org
> >=20
> > 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
> >=20
> >  arch/riscv/Kconfig             | 12 ++++----
> >  arch/riscv/Kconfig.erratas     |  6 ++--
> >  arch/riscv/include/asm/hwcap.h | 50 ++++++++++++++++++++--------------
> >  3 files changed, 38 insertions(+), 30 deletions(-)
> >=20
> > --=20
> > 2.39.2
> >
>=20
> LGTM, but if it was based on for-next then it could also immediately be
> applied to zicboz. For the series,

Hmm, I did it on top of fixes since this needs to go into v6.3.
Perhaps I can create a standalone patch for Zicboz and Palmer could merge
these two into both fixes & for-next, with the standalone applied on
top?


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