[PATCH 1/1] RISC-V: Add parameter to unaligned access speed

[Evan Green]

On Mon, Aug 5, 2024 at 11:48 AM Charlie Jenkins <charlie at rivosinc.com> wrote:
>
> On Mon, Aug 05, 2024 at 11:38:23AM -0700, Evan Green wrote:
> > On Mon, Aug 5, 2024 at 10:38 AM Jesse Taube <jesse at rivosinc.com> wrote:
> > >
> > > Add a kernel parameter to the unaligned access speed. This allows
> > > skiping of the speed tests for unaligned accesses, which often is very
> > > slow.
> > >
> > > Signed-off-by: Jesse Taube <jesse at rivosinc.com>
> >
> > How come this is a command line parameter rather than a Kconfig
> > option? I could be wrong, so I'll lay out my rationale and people can
> > pick it apart if I've got a bad assumption.
> >
> > I think of commandline parameters as (mostly) something end users
> > twiddle with, versus kconfig options as something system builders set
> > up. I'd largely expect end users not to notice two ticks at boot time.
> > I'd expect its system builders and fleet managers, who know their
> > hardware and build their kernels optimized for it, are the ones who
> > would want to shave off this time and go straight to the known answer.
> > Anecdotally, at ChromeOS we had a strong preference for Kconfig
> > options, as they were easier to compose and maintain than a loose pile
> > of commandline arguments.
> >
> > The commit text doesn't go into the rationale, intended audience, or
> > expected usage, so maybe my guesses miss the mark on what you're
> > thinking.
> > -Evan
>
> There was a brief discussion about this on Jesse's series about vector
> unaligned support [1]. The original idea was to use Zicclsm to allow
> people to set the unaligned access speed on pre-compiled distro kernels.
> However Zicclsm isn't useful so the alternative route was to use a
> kernel arg. There is already support for a Kconfig, the kernel arg is
> just another option for users.
>
> Link:
> https://lore.kernel.org/lkml/af3152b6-adf7-40fa-b2a1-87e66eec45b0@rivosinc.com/
> [1]

Ah got it, thanks for the explanation Charlie! If there are consumers
for this then the concept seems fine with me.
-Evan