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Re: [Xen-devel] [PATCH] x86/flush: use APIC ALLBUT destination shorthand when possible


  • To: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 3 Jan 2020 12:24:45 +0000
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  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Wei Liu <wl@xxxxxxx>
  • Delivery-date: Fri, 03 Jan 2020 12:25:02 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 03/01/2020 12:08, Jan Beulich wrote:
> On 24.12.2019 13:44, Roger Pau Monne wrote:
>> @@ -227,14 +233,47 @@ void flush_area_mask(const cpumask_t *mask, const void 
>> *va, unsigned int flags)
>>      if ( (flags & ~FLUSH_ORDER_MASK) &&
>>           !cpumask_subset(mask, cpumask_of(cpu)) )
>>      {
>> +        bool cpus_locked = false;
>> +
>>          spin_lock(&flush_lock);
>>          cpumask_and(&flush_cpumask, mask, &cpu_online_map);
>>          cpumask_clear_cpu(cpu, &flush_cpumask);
>>          flush_va      = va;
>>          flush_flags   = flags;
>> -        send_IPI_mask(&flush_cpumask, INVALIDATE_TLB_VECTOR);
>> +
>> +        /*
>> +         * Prevent any CPU hot{un}plug while sending the IPIs if we are to 
>> use
>> +         * a shorthand, also refuse to use a shorthand if not all CPUs are
>> +         * online or have been parked.
>> +         */
>> +        if ( system_state > SYS_STATE_smp_boot && !cpu_overflow &&
>> +             (cpus_locked = get_cpu_maps()) &&
>> +             (park_offline_cpus ||
> Why is it relevant whether we park offline CPUs, or whether we've
> even brought up all of the ones a system has? An IPI, in particular
> a broadcast one, shouldn't have any issue getting delivered if some
> of the nominal recipients don't listen, should it?

TGLX had a hard time time making shorthands work correctly on Linux. 
ISTR target CPUs in wait-for-SIPI may cause the source side of the IPI
to hang indefinitely, or yield an APIC Send error.

Linux maintains a bitmap of "CPUs which have booted once and we know can
safely handle IPIs", and doesn't permit any broadcast shorthands until
this matches the cpus_available mask.

~Andrew

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