Re: [PATCH v2 0/4] unified way to use static key and optimize pgtable_l4_enabled

[Jisheng Zhang <jszhang@kernel.org>]

On Mon, May 09, 2022 at 10:07:16AM +0530, Anup Patel wrote:
> On Sun, May 8, 2022 at 9:46 PM Jisheng Zhang <jszhang@kernel.org> wrote:
> >
> > Currently, riscv has several features which may not be supported on all
> > riscv platforms, for example, FPU, SV48, SV57 and so on. To support
> > unified kernel Image style, we need to check whether the feature is
> > suportted or not. If the check sits at hot code path, then performance
> > will be impacted a lot. static key can be used to solve the issue. In
> > the past, FPU support has been converted to use static key mechanism.
> > I believe we will have similar cases in the future. For example, the
> > SV48 support can take advantage of static key[1].
> >
> > patch1 is a simple W=1 warning fix.
> > patch2 introduces an unified mechanism to use static key for riscv cpu
> > features.
> > patch3 converts has_cpu() to use the mechanism.
> > patch4 uses the mechanism to optimize pgtable_l4|[l5]_enabled.
> >
> > [1] http://lists.infradead.org/pipermail/linux-riscv/2021-December/011164.html
> 
> Overall, using a script to generate CPU capabilities seems a bit
> over-engineered to me. We already have RISC-V ISA extension

Not all riscv features are *ISA* extensions. For example, SV48 and SV57
are not ISA extensions. IIRC, I asked this question before, here are
Atish's comments:

https://lore.kernel.org/linux-riscv/CAHBxVyF65jC_wvxcD6bueqpCY8-Kbahu1yxsSoBmO1s15dGkSQ@mail.gmail.com/

> parsing infrastructure which can be easily extended to support
> static key arrays.
>