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

[Uros Bizjak <ubizjak@gmail.com>]

On Tue, Oct 10, 2023 at 8:52 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Tue, 10 Oct 2023 at 11:41, Uros Bizjak <ubizjak@gmail.com> wrote:
> >
> > Yes, but does it CSE the load from multiple addresses?
>
> Yes, it should do that just right, because the *asm* itself is
> identical, just the offsets (that gcc then adds separately) would be
> different.

Indeed. To illustrate the question with an example, foo() and bar()
should compile to the same assembly, and there should be only one read
form m resp. n:

--cut here--
__seg_gs int m;

int foo (void)
{
  return m + m;
}

int n;

static inline int get (int *m)
{
  int res;

  asm ("mov %%gs:%1, %0" : "=r"(res) : "m"(*m));
  return res;
}

int bar (void)
{
  return get (&n) + get (&n);
}
--cut here--

And they do:

0000000000000000 <foo>:
  0:   65 8b 05 00 00 00 00    mov    %gs:0x0(%rip),%eax        # 7 <foo+0x7>
  7:   01 c0                   add    %eax,%eax
  9:   c3                      retq

0000000000000010 <bar>:
 10:   65 8b 05 00 00 00 00    mov    %gs:0x0(%rip),%eax        # 17 <bar+0x7>
 17:   01 c0                   add    %eax,%eax
 19:   c3                      retq

>
> This is not unlike how we depend on gcc CSE'ing the "current" part
> when doing multiple accesses of different members off that:
>
>     static __always_inline struct task_struct *get_current(void)
>     {
>         return this_cpu_read_stable(pcpu_hot.current_task);
>     }
>
> with this_cpu_read_stable() being an inline asm that lacks the memory
> component (the same way the fallback hides it by just using
> "%%gs:this_cpu_off" directly inside the asm, instead of exposing it as
> a memory access to gcc).
>
> Of course, I think that with the "__seg_gs" patches, we *could* expose
> the "%%gs:this_cpu_off" part to gcc, since gcc hopefully then can do
> the alias analysis on that side and see that it can CSE the thing
> anyway.
>
> That might be a better choice than __FORCE_ORDER, in fact.
>
> IOW, something like
>
>     static __always_inline unsigned long new_cpu_offset(void)
>     {
>         unsigned long res;
>         asm(ALTERNATIVE(
>                         "movq " __percpu_arg(1) ",%0",
>                         "rdgsbase %0",
>                         X86_FEATURE_FSGSBASE)
>                 : "=r" (res)
>                 : "m" (this_cpu_off));
>         return res;
>     }
>
> would presumably work together with your __seg_gs stuff.

I have zero experience with rdgsbase insn, but the above is not
dependent on __seg_gs, so (the movq part at least) would also work in
the current mainline. To work together with __seg_gs stuff,
this_cpu_offset should be enclosed in __my_cpu_var. Also, if rdgsbase
is substituted with rdfsbase, it will also work for 32-bit targets.

Uros.

> UNTESTED!!
>
>             Linus