Re: [Xen-devel] [PATCH 2/4] xen/x86: Drop erronious barriers

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 5 Dec 2016, Andrew Cooper wrote:
> None of these barriers serve any purpose, as they are not synchronising with
> any anything on remote CPUs.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
> 
> Restricting to just the $ARCH maintainers, as this is a project-wide sweep.
> 
> Julien/Stefano: I think the ARM smpboot inhereted the erronious barrier usage
> from x86, but I don't know whether further development has gained a dependence
> on them.

We turned them into smp_wmb already (kudos to IanC).


>  xen/arch/x86/acpi/cpu_idle.c  | 2 --
>  xen/arch/x86/cpu/mcheck/mce.c | 1 -
>  xen/arch/x86/crash.c          | 3 ---
>  xen/arch/x86/smpboot.c        | 2 --
>  4 files changed, 8 deletions(-)
> 
> diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
> index f36b184..09c8848 100644
> --- a/xen/arch/x86/acpi/cpu_idle.c
> +++ b/xen/arch/x86/acpi/cpu_idle.c
> @@ -1331,8 +1331,6 @@ void cpuidle_disable_deep_cstate(void)
>              max_cstate = 1;
>      }
>  
> -    mb();
> -
>      hpet_disable_legacy_broadcast();
>  }
>  
> diff --git a/xen/arch/x86/cpu/mcheck/mce.c b/xen/arch/x86/cpu/mcheck/mce.c
> index 2695b0c..460e336 100644
> --- a/xen/arch/x86/cpu/mcheck/mce.c
> +++ b/xen/arch/x86/cpu/mcheck/mce.c
> @@ -86,7 +86,6 @@ static x86_mce_vector_t _machine_check_vector = 
> unexpected_machine_check;
>  void x86_mce_vector_register(x86_mce_vector_t hdlr)
>  {
>      _machine_check_vector = hdlr;
> -    wmb();
>  }
>  
>  /* Call the installed machine check handler for this CPU setup. */
> diff --git a/xen/arch/x86/crash.c b/xen/arch/x86/crash.c
> index f28f527..4cadb5e 100644
> --- a/xen/arch/x86/crash.c
> +++ b/xen/arch/x86/crash.c
> @@ -146,9 +146,6 @@ static void nmi_shootdown_cpus(void)
>      write_atomic((unsigned long *)__va(__pa(&exception_table[TRAP_nmi])),
>                   (unsigned long)&do_nmi_crash);
>  
> -    /* Ensure the new callback function is set before sending out the NMI. */
> -    wmb();
> -
>      smp_send_nmi_allbutself();
>  
>      msecs = 1000; /* Wait at most a second for the other cpus to stop */
> diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
> index 3a9dd3e..0aa377a 100644
> --- a/xen/arch/x86/smpboot.c
> +++ b/xen/arch/x86/smpboot.c
> @@ -346,7 +346,6 @@ void start_secondary(void *unused)
>      spin_debug_enable();
>      set_cpu_sibling_map(cpu);
>      notify_cpu_starting(cpu);
> -    wmb();
>  
>      /*
>       * We need to hold vector_lock so there the set of online cpus
> @@ -364,7 +363,6 @@ void start_secondary(void *unused)
>  
>      microcode_resume_cpu(cpu);
>  
> -    wmb();
>      startup_cpu_idle_loop();
>  }
>  
> -- 
> 2.1.4
> 

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