On 13/04/2017 22:09, Radim Krcmar wrote: > KVM PIT (arch/x86/kvm/i8254.c) uses it to keep track of lost edge > interrupts so it could re-inject them. > Windows needed it for reasonable timeflow, IIRC. This is for old Linux. Windows needs the RTC tracking, which also uses the irq ack notifier. > The feature might have been introduced because some OS assumed presence > of the feature if x2APIC was available ... > Right, reverting it is pretty safe, because it was broken before. > (And we ignore the SPIV completely when forwarding EOI to userspace.) Yeah, that's the important part. I'm a bit worried of the consequences for migration, but AIUI you think it's fine? (That is, too many EOIs are not a problem, only too few are). Paolo
