Am 15.06.2015 um 17:18 schrieb Peter Maydell:
> On 15 June 2015 at 16:07, Alex Züpke <alexander.zue...@hs-rm.de> wrote:
>> Am 15.06.2015 um 17:04 schrieb Peter Maydell:
>>> On 12 June 2015 at 17:38, Alex Züpke <alexander.zue...@hs-rm.de> wrote:
>>>> Hi,
>>>>
>>>> I'm benchmarking some IPI (== inter-processor-interrupt) synchronization
>>>> stuff of my custom kernel on QEMU ARM (qemu-system-arm -M vexpress-a15
>>>> -smp 2) and ran into the following problem: pending IPIs are delayed until
>>>> the QEMU main loop receives an event (for example the timer interrupt
>>>> expires or I press a key on the console).
>>>>
>>>> The following timing diagram tries to show this:
>>>>
>>>> CPU #0 CPU #1
>>>> ====== ======
>>>> ... other stuff ... WFI (wait for interrupt, like x86 "HLT")
>>>> send SGI in MPCore
>>>> polls for completeness
>>>> <time passes ...>
>>>> polls ...
>>>> <... and passes ...>
>>>> still polls ...
>>>> <... and passes ...>
>>>> still polls ...
>>>> <... and passes ...>
>>>>
>>>>
>>>> <timer interrupt expires>
>>>> <now QEMU switches to CPU #1>
>>>> receives IPI
>>>> signals completeness
>>>> WFI
>>>> <QEMU switches to CPU #0>
>>>> polling done
>>>> process timer interrupt
>>>> ...
>>>
>>> Does your polling loop have a YIELD insn in it? We (and hardware)
>>> can use that as a hint that you're busy-looping and we should
>>> try doing something else. (QEMU doesn't implement that for A32/T32
>>> yet, but we should; we already do on A64.)
>>
>> Yes, I should be yielding here, but SEV isn't implemented.
>> Probably the notification should be done there as well.
>
> YIELD isn't related to SEV -- it's just a generic "hey, I'm
> polling" hint. We NOP SEV, and make WFE be a "yield this CPU's
> timeslice" event", which is architecturally valid, and sufficient
> for this situation anyway.
>
> -- PMM
>
Ah, YIELD, thanks for the hint. I never used it because all CPUs I got my hand
on were physical ones so far ...
So this is the way to go:
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -4084,6 +4084,7 @@ static void gen_nop_hint(DisasContext *s, int val)
gen_set_pc_im(s, s->pc);
s->is_jmp = DISAS_WFI;
break;
+ case 1: /* yield */
case 2: /* wfe */
gen_set_pc_im(s, s->pc);
s->is_jmp = DISAS_WFE;
Thanks
Alex
