Re: [PATCH] x86/shutdown: mask LVTERR ahead of offlining CPUs

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 19/09/2024 4:27 pm, Roger Pau Monne wrote:
> Leaving active interrupt sources targeting APIC IDs that are offline can be
> problematic on AMD machines during shutdown.

What exactly qualifies as "offline" here?

We don't self-INIT, so I'm guessing we leave the APIC in some kind of
disabled state, especially given ...

>   This is due to AMD local APICs
> reporting Receive Accept Errors when a message is not handled by any APIC on
> the system.

... this.


>   Note Intel SDM states that Receive Accept Errors are only reported
> on P6 family and Pentium processors.
>
> If at shutdown an active interrupt source is left targeting an offline APIC 
> ID,
> the following can be seen on AMD boxes:
>
> Hardware Dom0 shutdown: rebooting machine
> APIC error on CPU0: 00(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> APIC error on CPU0: 08(08), Receive accept error
> [...]
>
> Thus preventing the shutdown.  In the above case the interrupt source that was
> left targeting an offline APIC ID was the serial console one

While masking LVTERR might allow more progress, it's not a wise approach.

The real issue here is that the UART driver is still active as we're
trying to tear the system down.  If nothing else, it's rude to leave an
active interrupt source for the kexec kernel to deal with.

IMO, we should shut the UART down like other devices, and move it back
into polled mode.

> , so printing of
> the local APIC ESR lead to more unhandled messages on the APIC bus, leaving 
> the
> host unable to make progress.

Minor note, but there's not been an APIC bus in decades.  Here, I'd
simply say "lead to more console IRQs, and more errors".

>
> Mask LVTERR ahead of bringing any CPU offline, thus avoiding receiving
> interrupts for any APIC reported errors.  Note that other local APIC errors
> will also be ignored.  At the point where the masking is done it's unlikely 
> for
> any reported APIC errors to be meaningful anyway, the system is about to 
> reboot
> or power off.
>
> The LVTERR masking could be limited to AMD, but there's no guarantee that in
> the future Intel parts also start reporting the error, thus hitting the same
> issue.  Unifying behavior across vendors when possible seems more desirable.
> The local APIC gets wholesale disabled as part of offlining the CPUs and
> bringing the system down in __stop_this_cpu().
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Note a similar issue possibly exists in the nmi_shootdown_cpus() path, however
> that being a crash path it is more complicated to uniformly mask the LVTERR
> strictly ahead of offlining CPUs.  That path is also more resilient AFAICT, as
> nmi_shootdown_cpus() disables interrupts at the start (so no LVTERR interrupt
> will be handled) and the CPUs are stopped using an NMI, which would bypass any
> LVTERR processing.
> ---
>  xen/arch/x86/smp.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/xen/arch/x86/smp.c b/xen/arch/x86/smp.c
> index 04c6a0572319..399ec7491ac6 100644
> --- a/xen/arch/x86/smp.c
> +++ b/xen/arch/x86/smp.c
> @@ -348,6 +348,11 @@ static void cf_check stop_this_cpu(void *dummy)
>          halt();
>  }
>  
> +static void cf_check mask_lvterr(void *dummy)
> +{
> +    apic_write(APIC_LVTERR, ERROR_APIC_VECTOR | APIC_LVT_MASKED);
> +}
> +
>  /*
>   * Stop all CPUs and turn off local APICs and the IO-APIC, so other OSs see 
> a 
>   * clean IRQ state.
> @@ -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?

~Andrew