Re: [PATCH RFC 00/28] ARM: Switch to generic entry

[Linus Walleij]

On Thu, Oct 10, 2024 at 1:55 PM Russell King (Oracle)
<linux@xxxxxxxxxxxxxxx> wrote:

> I haven't looked at the series yet, but I guess we're throwing away
> all the effort I put in to make stuff like syscalls as fast as
> possible.
>
> So the question is... do we want performance, or do we want generic
> (and slower) code?

Yes, the very same question that came to me as I was working on it, we
need to reach some conclusion here. Al Viro also put some nice
assembly optimizations in the syscall restart that just go out the
window as well.

Some of the C interpersing relates to the RCU context tracking that
really likes to be called at every single IRQ, FIQ or SWI, and where ARM32
is one of the few last users of the user_exit_callable()/user_enter_callable()
API which is obviously less intrusive as it only needs to get called
at transitions to/from userspace, while these calls are marked
with big block letters as obsolete in the context tracker.

> It seems insane to me that we spend time micro-optimising things like
> memcpy, memset, divide routines, but then go and throw away performance
> that applications actually rely upon, such as syscall performance.

Yes, this series is a real RFC in the true sense of the word.

Yours,
Linus Walleij