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    SubjectRe: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
    On Wed, Oct 18, 2023 at 8:26 PM Uros Bizjak <ubizjak@gmail.com> wrote:
    >
    > On Wed, Oct 18, 2023 at 8:16 PM Linus Torvalds
    > <torvalds@linux-foundation.org> wrote:
    > >
    > > On Wed, 18 Oct 2023 at 11:08, Uros Bizjak <ubizjak@gmail.com> wrote:
    > > >
    > > > But loads from non-const memory work like the above.
    > >
    > > Yes, I'm certainly ok with the move to use plain loads from __seg_gs
    > > for the percpu accesses. If they didn't honor the memory clobber, we
    > > could never use it at all.
    > >
    > > I was just saying that the 'const' alias trick isn't useful for
    > > anything else than 'current', because everything else needs to at
    > > least honor our existing barriers.
    >
    > FYI, smp_processor_id() is implemented as:
    >
    > #define __smp_processor_id() __this_cpu_read(pcpu_hot.cpu_number)
    >
    > where __this_* forces volatile access which disables CSE.
    >
    > *If* the variable is really stable, then it should use __raw_cpu_read.
    > Both, __raw_* and __this_* were recently (tip/percpu branch)
    > implemented for SEG_SUPPORT as:

    This pach works for me:

    --cut here--
    diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
    index 4fab2ed454f3..6eda4748bf64 100644
    --- a/arch/x86/include/asm/smp.h
    +++ b/arch/x86/include/asm/smp.h
    @@ -141,8 +141,7 @@ __visible void
    smp_call_function_single_interrupt(struct pt_regs *r);
    * This function is needed by all SMP systems. It must _always_ be valid
    * from the initial startup.
    */
    -#define raw_smp_processor_id() this_cpu_read(pcpu_hot.cpu_number)
    -#define __smp_processor_id() __this_cpu_read(pcpu_hot.cpu_number)
    +#define raw_smp_processor_id() raw_cpu_read(pcpu_hot.cpu_number)

    #ifdef CONFIG_X86_32
    extern int safe_smp_processor_id(void);
    --cut here--

    But removes merely 10 reads from 3219.
    BTW: I also don't understand the comment from include/linux/smp.h:

    /*
    * Allow the architecture to differentiate between a stable and unstable read.
    * For example, x86 uses an IRQ-safe asm-volatile read for the unstable but a
    * regular asm read for the stable.
    */
    #ifndef __smp_processor_id
    #define __smp_processor_id(x) raw_smp_processor_id(x)
    #endif
    All reads up to word size on x86 are atomic, so IRQ safe. asm-volatile
    is not some IRQ property, but prevents the compiler from CSE the asm
    and scheduling (moving) asm around too much.

    Uros.

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