Re: [PATCH 4/4] x86/percpu: Use C for percpu read/write accessors

[Linus Torvalds]

On Sun, 8 Oct 2023 at 12:18, Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>
> Let me see what happens here. I have changed *only* raw_cpu_read_8,
> but the GP fault is reported in cpu_init_exception_handling, which
> uses this_cpu_ptr. Please note that all per-cpu initializations go
> through existing {this|raw}_cpu_write.

I think it's an ordering issue, and I think you may hit some issue
with loading TR od the GDT or whatever.

For example, we have this

        set_tss_desc(cpu, &get_cpu_entry_area(cpu)->tss.x86_tss);

followed by

        asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));

in native_load_tr_desc(), and I think we might want to add a "memory"
clobber to it to make sure it is serialized with any stores to the GDT
entries in question.

I don't think *that* particular thing is the issue (because you kept
the writes as-is and still hit things), but I think it's an example of
some lazy inline asm constraints that could possibly cause problems if
the ordering changes.

And yes, this code ends up depending on things like
CONFIG_PARAVIRT_XXL for whether it uses the native TR loading or uses
some paravirt version, so config options can make a difference.

Again: I don't think it's that "ltr" instruction. I'm calling it out
just as a "that function does some funky things", and the load TR is
*one* of the funky things, and it looks like it could be the same type
of thing that then causes issues.

Things like CONFIG_SMP might also matter, because the percpu setup is
different. On UP, the *segment* use goes away, but I think the whole
"use inline asm vs regular memory ops" remains (admittedly I did *not*
verify that, I might be speaking out of my *ss).

Your dump does end up being close to a %gs access:

   0: 4a 03 04 ed 40 19 15 add    -0x7aeae6c0(,%r13,8),%rax
   7: 85
   8: 48 89 c7              mov    %rax,%rdi
   b: e8 9c bb ff ff        call   0xffffffffffffbbac
  10: 48 c7 c0 10 73 02 00 mov    $0x27310,%rax
  17: 48 ba 00 00 00 00 00 movabs $0xdffffc0000000000,%rdx
  1e: fc ff df
  21: 48 c1 e8 03          shr    $0x3,%rax
  25:* 80 3c 10 00          cmpb   $0x0,(%rax,%rdx,1) <-- trapping instruction
  29: 0f 85 21 05 00 00    jne    0x550
  2f: 65 48 8b 05 45 26 f6 mov    %gs:0x7ef62645(%rip),%rax        # 0x7ef6267c
  36: 7e
  37: 48 8d 7b 24          lea    0x24(%rbx),%rdi

but I don't know what the "call" before is, so I wasn't able to match
it up with any obvious code in there.

              Linus