Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

[Nadav Amit]

> On Oct 17, 2023, at 2:02 AM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> -       #APP
> -       movq    %gs:pcpu_hot(%rip), %r12
> -       #NO_APP
> -       testq   $16384, (%r12)
> +       testq   $16384, (%rdi)
> 
> so I think this is the right thing to do. I checked that the 32-bit
> code builds and looks sane too.
> 
> I do think the 'old/new' naming in the FPU code should probably be
> 'prev/next' to match the switch_to() naming, but I didn't do that.
> 
> Comments?

Yes, the FPU issue is the one that caused me to crash before. I indeed missed
the switch_fpu_prepare(). The other issue that I encountered before, with
__resctrl_sched_in() has already been taken care of.

It would have been nice to somehow prevent such a thing from reoccurring.
Presumably objtool could have ensured it is so. But anyhow, I do not know of
any other currently open issues.

This whole thing (in addition to Uros’s analysis and objdump numbers) show
that the const-alias allows much more aggressive optimizations than the
current this_cpu_read_stable().