Re: [PATCH 0/2] Xen real-time x86

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.07.2025 12:11, Roger Pau Monné wrote:
> On Mon, Jul 07, 2025 at 05:06:53PM -0700, Stefano Stabellini wrote:
>> Hi all,
>>
>> This short patch series improves Xen real-time execution on AMD x86
>> processors.
>>
>> The key to real-time performance is deterministic guest execution times
>> and deterministic guest interrupt latency. In such configurations, the
>> null scheduler is typically used, and there should be no IPIs or other
>> sources of vCPU execution interruptions beyond the guest timer interrupt
>> as configured by the guest, and any passthrough interrupts for
>> passthrough devices.
>>
>> This is because, upon receiving a critical interrupt, the guest (such as
>> FreeRTOS or Zephyr) typically has a very short window of time to
>> complete the required action. Being interrupted in the middle of this
>> critical section could prevent the guest from completing the action
>> within the allotted time, leading to malfunctions.
> 
> There's IMO still one pending issue after this series on x86, maybe
> you have addressed this with some local patch.

Not just one, I think. We use IPIs for other purposes as well. The way
I read the text above, all of them are a (potential) problem.

Jan

>  Interrupt forwarding
> from Xen into HVM/PVH guests uses a softirq to do the injection, which
> means there's a non-deterministic window of latency between when the
> interrupt is received by Xen, as to when it's injected to the guest,
> because the softirq might not get processed right after being set as
> pending (there might be other softirqs to process, or simply Xen might
> be busy doing some other operation).
> 
> I think you want to look into adding a new command line option or
> similar, that allows selecting whether guest IRQs are deferred to a
> softirq for injection, or are injected as part of the processing done
> in the IRQ handler itself.
> 
> Otherwise there will always be a non-deterministic amount of latency
> on x86 w.r.t. HVM/PVH passthrough guest interrupts.  Haven't you seen
> some weird/unexpected variance when doing this passthrough interrupt
> latency measurements on x86?
> 
> Regards, Roger.