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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.12.16 at 12:25, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 05/12/16 11:18, Jan Beulich wrote:
>>>>> On 05.12.16 at 11:05, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> --- 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).
> 
> This wmb() is already only a barrier() (Fixed in the final patch)

Good point.

> Even if it weren't, wrmsr doesn't interact with sfence, so the barrier
> would still be pointless.

Are you sure there's absolutely nothing in replacement for the lack
of serialization?

That said, we're still having enough of a barrier left anyway, due to
the ones in send_IPI_mask_x2apic_{phys,cluster}(), and due to the
LAPIC mapping being UC. So yes, I'm fine with dropping it here then.

>>> --- 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()).
> 
> Why?  It doesn't relate to an smp_rmb() on any other CPU, and is
> therefore wrong to use.

I think it does, just not with one that's spelled out as smp_rmb().
Instead __cpu_up() spins on !cpu_online(), using cpu_relax() as
a de-facto equivalent of smp_rmb().

Jan


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