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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.12.16 at 11:05, <andrew.cooper3@xxxxxxxxxx> wrote:

As to the title - don't you rather mean "pointless" or some such? It's
not really an error to have them where they are.

> --- 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();

I don't think I agree with this change - we certainly want to make
sure the APIC write happens only after after the exception vector
adjustment became visible, namely when in x2APIC mode (where
the respective WRMSRs are not serializing).

> --- 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

Hmm, this one is indeed redundant with the lock_vector_lock()
following right below, but if that lock wasn't there, I think it
would be needed to order set_cpu_sibling_map() and the
setting of the bit in the online map. So I think it would better
stay (but be relaxed to smb_wmb()).

Jan


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