On 23/04/2025 12:32 pm, Roger Pau Monne wrote:
> There are several errata on Intel regarding the usage of the MONITOR/MWAIT
> instructions, all having in common that stores to the monitored region
> might not wake up the CPU.
>
> Fix them by forcing the sending of an IPI for the affected models.
>
> The Ice Lake issue has been reproduced internally on XenServer hardware,
> and the fix does seem to prevent it. The symptom was APs getting stuck in
> the idle loop immediately after bring up, which in turn prevented the BSP
> from making progress. This would happen before the watchdog was
> initialized, and hence the whole system would get stuck.
>
> Signed-off-by: Roger Pau Monné <roger....@citrix.com>
> ---
> Apollo and Lunar Lake fixes have not been tested, due to lack of hardware.
Acked-by: Andrew Cooper <andrew.coop...@citrix.com>
> diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
> index 420198406def..1dbf15b01ed7 100644
> --- a/xen/arch/x86/acpi/cpu_idle.c
> +++ b/xen/arch/x86/acpi/cpu_idle.c
> @@ -441,8 +441,14 @@ void cpuidle_wakeup_mwait(cpumask_t *mask)
> cpumask_andnot(mask, mask, &target);
> }
>
> +/* Force sending of a wakeup IPI regardless of mwait usage. */
> +bool __ro_after_init force_mwait_ipi_wakeup;
> +
> bool arch_skip_send_event_check(unsigned int cpu)
> {
> + if ( force_mwait_ipi_wakeup )
> + return false;
> +
I don't especially like this. The callers are a loop over all CPUs, and
this can't be inlined/simplified automatically.
But, lets get the fix in place first. Optimising comes later.
~Andrew
