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.
