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Re: [PATCH] x86/shutdown: mask LVTERR ahead of offlining CPUs
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 23 Sep 2024 13:54:51 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Delivery-date: Mon, 23 Sep 2024 11:54:47 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 20.09.2024 10:35, Roger Pau Monné wrote:
> On Thu, Sep 19, 2024 at 10:19:49PM +0200, Andrew Cooper wrote:
>> On 19/09/2024 4:27 pm, Roger Pau Monne wrote:
>>> @@ -364,6 +369,18 @@ void smp_send_stop(void)
>>> fixup_irqs(cpumask_of(cpu), 0);
>>> local_irq_enable();
>>>
>>> + /*
>>> + * Mask the local APIC error vector ahead of stopping CPUs.
>>> + *
>>> + * On AMD the local APIC will report Receive Accept Errors if the
>>> + * destination APIC ID of an interrupt message is not online.
>>> There's
>>> + * no guarantee that fixup_irqs() will evacuate all interrupts -
>>> + * possibly because the sole CPU remaining online doesn't have
>>> enough
>>> + * vectors to accommodate all.
>>> + */
>>> + smp_call_function(mask_lvterr, NULL, true);
>>> + mask_lvterr(NULL);
>>> +
>>> smp_call_function(stop_this_cpu, NULL, 0);
>>
>> Irrespective of the question over approach, stop_this_cpu() should end
>> up clearing LVTERR. Why doesn't that suffice?
>
> No, because those are no ordered. The sequence needs to strictly be:
>
> - Mask LVTERR on all CPUs.
> <wait for masking to be done uniformly>
> - Stop CPUs.
>
> Otherwise CPUs might be stopped before LVTERR has been uniformly
> masked, leading to Receive accept error reported on the CPUs that
> don't yet have LVTERR masked.
Yet fixup_irqs() has moved everything to CPU0. Nothing should go to any of
the APs anymore. Fiddling with LVTERR here feels like curing a symptom
rather than the root cause.
Jan
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