On 27.02.2023 12:48, Simon Gaiser wrote: > PIT timer: During some previous private discussion it was mentioned that > the PIT timer that Xen initializes for IO-APIC testing prevents S0ix > residency and therefore that part needs to be reworked. But if I'm > reading the current code correctly Xen can already use the HPET timer > instead, either with an automatic fallback if PIT is unavailable or by > forcing it via hpet=legacy-replacement=1. Looking at the rest I think > the PIT isn't used if Xen finds another clocksource. Did I miss > something?
I think the concern was with calibrating the APIC clock against PIT, but as you say that connection has been cut. So I think there's no prereq work left there; Andrew may prove me wrong, though. > mwait idle driver: While mwait-idle.c share a lot of code with Linux's > intel_idle.c and the imported code seems to have been updated (for > example for Alder Lake) it only supports the CPUs with hardcoded > cstates. Linux's code also has a code path to read the cstate config > from ACPI if the driver doesn't has a hard coded config for the model. > This is needed for example for Tiger Lake. For my current testing I > added the values the Linux code reads from ACPI by hand. But that's of > course no proper solution. How should this be handled in Xen (IIUC some > ACPI things are handled by Xen and some by dom0)? Well, there's first of all some lack of understanding on my part: It hasn't become clear to me why Linux now has two CPU idle drivers consuming ACPI data (intel_idle and the purely ACPI-based one). If two are really needed, then I guess mwait-idle would need extending to also consume data passed up from Dom0, just like the ACPI-only driver in Xen does (and which doesn't do anything until such data was supplied). > I'm not sure yet is what else in Xen needs to learn about S0ix. Running > domains need to be halted, that can be handled by the toolstack. What > other Xen internal things need to be aware of S0ix? Like avoiding > unnecessary timer wakeups or similar. That's currently my biggest > unknown and it would be great if someone with more insight and overview > could give some hints here. I didn't think the higher-level-software side of things was different from S3. Instead I thought it's merely a matter of different interaction with hardware that's needed (which, as per above, includes avoiding certain things, i.e. in particular the PIT). Jan
