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

[Andrew Jones]

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@xxxxxxxxxx/T/#m21390d570997145d31dd8bb95002fd61f99c6573
> 
> CC: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
> CC: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> CC: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> CC: Heiko Stuebner <heiko.stuebner@xxxxxxxx>
> CC: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> CC: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
> CC: Jisheng Zhang <jszhang@xxxxxxxxxx>
> CC: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> CC: Jason A. Donenfeld <Jason@xxxxxxxxx>
> CC: linux-riscv@xxxxxxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> 
> 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@xxxxxxxxxxxxxxxx>

Thanks,
drew