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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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