Re: [PATCH v4 1/3] xen/x86: add nmi continuation framework

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.11.2020 10:50, Juergen Gross wrote:
> Actions in NMI context are rather limited as e.g. locking is rather
> fragile.
> 
> Add a framework to continue processing in normal interrupt context
> after leaving NMI processing.
> 
> This is done by a high priority interrupt vector triggered via a
> self IPI from NMI context, which will then call the continuation
> function specified during NMI handling.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one further adjustment request:

> @@ -1799,6 +1800,24 @@ void unset_nmi_callback(void)
>      nmi_callback = dummy_nmi_callback;
>  }
>  
> +bool nmi_check_continuation(void)
> +{
> +    bool ret = false;
> +
> +    return ret;
> +}
> +
> +void trigger_nmi_continuation(void)
> +{
> +    /*
> +     * Issue a self-IPI. Handling is done in spurious_interrupt().
> +     * NMI could have happened in IPI sequence, so wait for ICR being idle
> +     * again before leaving NMI handler.
> +     */
> +    send_IPI_self(SPURIOUS_APIC_VECTOR);
> +    apic_wait_icr_idle();
> +}

This additionally relies on send_IPI_self_legacy() calling
send_IPI_shortcut(), rather than e.g. resolving the local CPU
number to a destination ID. I think this wants saying maybe
here, but more importantly in that function.

Jan