On 03/07/2025 10:43 am, Jan Beulich wrote:
> On 02.07.2025 16:41, Andrew Cooper wrote:
>> @@ -461,12 +468,19 @@ void mwait_idle_with_hints(unsigned int eax, unsigned
>> int ecx)
>>
>> monitor(this_softirq_pending, 0, 0);
>>
>> + ASSERT(!local_irq_is_enabled());
>> +
>> if ( !*this_softirq_pending )
>> {
>> struct cpu_info *info = get_cpu_info();
>>
>> spec_ctrl_enter_idle(info);
>> - mwait(eax, ecx);
>> +
>> + if ( ecx & MWAIT_ECX_INTERRUPT_BREAK )
>> + mwait(eax, ecx);
>> + else
>> + sti_mwait_cli(eax, ecx);
> Actually, I'm curious: It seems quite likely that you did consider an
> alternative resulting in assembly code like this:
>
> test $MWAIT_ECX_INTERRUPT_BREAK, %cl
> jz 0f
> sti
> 0:
> monitor
> cli
>
> CLI being a relatively cheap operation aiui, is there anything wrong or
> undesirable with this (smaller overall) alternative?
Other than it needing to be mwait? The overheads aren't interesting;
they're nothing compared to going idle.
What does matter is that such a transformation cannot exist in mwait()
itself, as that breaks acpi_dead_idle(), and if we turn this mwait()
into inline asm, there's only a single caller of mwait() left.
~Andrew
