[PATCH 7/9] riscv: Prepare for unaligned access type table lookups

[Andrew Jones]

On Mon, Feb 10, 2025 at 12:07:40PM +0100, Clément Léger wrote:
> 
> 
> On 10/02/2025 11:16, Anup Patel wrote:
> > On Sat, Feb 8, 2025 at 6:53 AM Charlie Jenkins <charlie at rivosinc.com> wrote:
> >>
> >> On Fri, Feb 07, 2025 at 05:19:47PM +0100, Andrew Jones wrote:
> >>> Probing unaligned accesses on boot is time consuming. Provide a
> >>> function which will be used to look up the access type in a table
> >>> by id registers. Vendors which provide table entries can then skip
> >>> the probing.
> >>
> >> The access checker in my experience is only time consuming on slow
> >> hardware. Hardware that supports fast unaligned accesses isn't really
> >> impacted by this? Avoiding a list of hardware that has slow/fast
> >> unaligned accesses in the kernel was the main reason for dynamically
> >> checking. We did introduce the config option to compile the kernel with
> >> assumed slow/fast accesses, which of course has the downside of
> >> recompiling the kernel and I assume that you already considered that.
> > 
> > The kconfig option does not align with the vision of running the same
> > kernel image across platforms.
> 
> I'd would be advocating to remove compile time options as well and use
> another way to skip the probe (see below).
> 
> > 
> >>
> >> Instead of having a table in the kernel, something that would be more
> >> platform agnostic would be to have an extension that signals this
> >> information. That seems like it would accomplish the same goal and
> >> leverage the existing infrastructure in the kernel, albeit with the need
> >> to make a new extension.
> >>
> > 
> > IMO, expecting an ISA extension to be defined for all possible
> > microarchitectural choices is not going to scale so it is better
> > to have infrastructure in kernel itself to infer microarchitectural
> > choices based on RISC-V implementation ID.
> 
> Since adding an extension seems quite unlikely, and that a device-tree
> property is likely DT centric and not applicable to ACPI as well, was a
> command line argument considered ?
>

I did consider adding a command line option in addition to the table,
allowing platforms which neither have a table entry [yet] nor want to do
the speed test, to set whatever they like. In the end, I dropped it, since
I don't have a use case at this time. However, if we really don't want a
table, then I can look into the command line option instead.

Thanks,
drew