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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.)

Jan

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