Messages in this thread | | | From | Linus Torvalds <> | Date | Sat, 7 Sep 2019 12:27:42 -0700 | Subject | Re: Linux 5.3-rc7 |
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On Sat, Sep 7, 2019 at 12:17 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > I'm really not clear on why it's a good idea to clear the LDR bits on > shutdown, and commit 558682b52919 ("x86/apic: Include the LDR when > clearing out APIC registers") just looks pointless. And now it has > proven to break some machines. > > So why wouldn't we just revert it?
Side note: looking around for the discussion about this patch, at least one version of the patch from Bandan had
+ if (!x2apic_enabled) {
rather than
+ if (!x2apic_enabled()) {
which meant that whatever Bandan tested at that point was actually a complete no-op, since "!x2apic_enabled" is never true (it tests a function pointer against NULL, which it won't be).
Then that was fixed by the time it hit -tip (and eventually my tree), but it kind of shows how the patch history of this is all questionable. Further strengthened by a quote from that discussion:
"this is really a KVM bug but it doesn't hurt to clear out the LDR in the guest and then, it wouldn't need a hypervisor fix"
and clearly it *does* hurt to clear the LDR in the guest, making the whole thinking behind the patch wrong and broken. The kernel clearly _does_ depend on LDR having the right contents.
Now, I still suspect the boot problem then comes from our cpu0_logical_apicid use mentioned in that previous email, but at this point I think the proper fix is "revert for now, and we can look at this as a cleanup with the cpu0_logical_apicid thing for 5.4 instead".
Hmm?
Linus
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