[PATCH v2 1/2] riscv: introduce RISCV_EFFICIENT_UNALIGNED_ACCESS

[Eric Biggers]

On Mon, Dec 04, 2023 at 11:15:28AM -0800, Charlie Jenkins wrote:
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 7f8aa25457ba..0a76209e9b02 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -654,6 +654,18 @@ config RISCV_MISALIGNED
> >  	  load/store for both kernel and userspace. When disable, misaligned
> >  	  accesses will generate SIGBUS in userspace and panic in kernel.
> >  
> > +config RISCV_EFFICIENT_UNALIGNED_ACCESS
> 
> There already exists hwprobe for this purpose. If kernel code wants to
> leverage the efficient unaligned accesses of hardware, it can use static
> keys. I have a patch that will set this static key if the hardware was
> detected to have fast unaligned accesses:
> 
> https://lore.kernel.org/linux-riscv/20231117-optimize_checksum-v11-2-7d9d954fe361@rivosinc.com/

Is the plan to make the get_unaligned* and put_unaligned* macros expand to code
for both cases, and select between them using a static key?  Note that there are
a very large number of callers of these macros in the kernel.  And what about
kernel code that checks CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS directly?

AFAIK, no other Linux architecture supports kernel images where the unaligned
access support is unknown at compile time.  It's not clear to me that such an
approach is feasible.  A static key can easily be provided, but it's unclear
what code would use it, given that currently lots of kernel code assumes that
unaligned access support is known at compile time.

Meanwhile, there are people building kernels they know will only be deployed on
systems where unaligned accesses are supported.  To me, it seems useful to
provide a kconfig option for them to build a more efficient kernel.

- Eric