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: <20230324114311.qjz62iofsrhgdlnz@orel>
Date:   Fri, 24 Mar 2023 12:43:11 +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 11:37:05AM +0000, Conor Dooley wrote:
> 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.
> > > 
> > > 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,
> 
> Hmm, I did it on top of fixes since this needs to go into v6.3.

Ah, sure.

> 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?

Sounds good.

Thanks,
drew