[PATCH 0/4] riscv: uaccess: optimizations

[Linus Torvalds]

On Tue, 25 Jun 2024 at 00:22, Arnd Bergmann <arnd at arndb.de> wrote:
>
> I think the only bets we really need from an architecture
> here are:
>
> - __enable_user_access()/__disable_user_access() in
>    the label version

KCSAN wants user_access_save/restore() too, but yes.

> - __raw_get_mem_{1,2,4,8}() and __raw_put_mem_{1,2,4,8}()

You still need to split it into user/kernel.

Yes, *often* there is just one address space and they can be one "mem"
accessor thing, but we do have architectures with actually separate
user and kernel address spaces.

But yes, it would be lovely if we did things as "implement the
low-level accessor functions and we'll wrap them for the generic case"
rather than have every architecture have to do the wrapping..

The wrapping isn't just the label-based "unsafe" accesses and the
traditional error number return case, it's also the size-based case
statements ("poor man's generics").

The problem, of course, is that the majority of architectures are
based on the legacy interfaces, so it's a lot of annoying low-level
small details. I think there's a reason why nobody did the arm64
"unsafe" ones - the patch didn't end up being that bad, but it's just
fairly grotty code.

But apparently the arm64 patch was simple enough that at least RISC-V
people went "that doesn't look so bad".

Maybe other architectures will also be fairly straightforward when
there's a fairly clear example of how it should be done.

             Linus