Re: [Xen-devel] [PATCH v9 15/15] microcode: block #NMI handling when loading an ucode

[Chao Gao <chao.gao@xxxxxxxxx>]

On Thu, Aug 29, 2019 at 02:22:47PM +0200, Jan Beulich wrote:
>On 19.08.2019 03:25, Chao Gao wrote:
>> @@ -481,12 +478,28 @@ static int do_microcode_update(void *patch)
>>      return ret;
>>  }
>>  
>> +static int microcode_nmi_callback(const struct cpu_user_regs *regs, int cpu)
>> +{
>> +    /* The first thread of a core is to load an update. Don't block it. */
>> +    if ( cpu == cpumask_first(per_cpu(cpu_sibling_mask, cpu)) ||
>> +         loading_state != LOADING_CALLIN )
>> +        return 0;
>> +
>> +    cpumask_set_cpu(cpu, &cpu_callin_map);
>> +
>> +    while ( loading_state != LOADING_EXIT )
>> +        cpu_relax();
>> +
>> +    return 0;
>> +}
>
>By returning 0 you tell do_nmi() to continue processing the NMI.
>Since you can't tell whether a non-IPI NMI has surfaced at about
>the same time this is generally the right thing imo, but how do
>you prevent unknown_nmi_error() from getting entered when do_nmi()
>ends up setting handle_unknown to true? (The question is mostly
>rhetorical, but there's a disconnect between do_nmi() checking
>"cpu == 0" and the control thread running on
>cpumask_first(&cpu_online_map), i.e. you introduce a well hidden
>dependency on CPU 0 never going offline. IOW my request is to at
>least make this less well hidden, such that it can be noticed if
>and when someone endeavors to remove said limitation.)

Seems the issue is that we couldn't send IPI NMI to BSP, otherwise
unknown_nmi_error() would be trigger. And loading ucode after
rendezvousing all CPUs in NMI handler expects all CPUs to receive IPI
NMI. So this approach always has such issue.

Considering self_nmi is called at another place, could we provide a
way to temporarily suppress or (force) ignore unknown nmi error?

Thanks
Chao

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