On Tue, 16 Jan 2018, Vitaly Kuznetsov wrote: > Hyper-V supports Live Migration notification. This is supposed to be used > in conjunction with TSC emulation: when we are migrated to a host with > different TSC frequency for some short period host emulates our accesses > to TSC and sends us an interrupt to notify about the event. When we're > done updating everything we can disable TSC emulation and everything will > start working fast again. > > We didn't need these notifications before as Hyper-V guests are not > supposed to use TSC as a clocksource: in Linux we even mark it as unstable > on boot. Guests normally use 'tsc page' clocksouce and host updates its > values on migrations automatically. > > Things change when we want to run nested virtualization: even when we pass > through PV clocksources (kvm-clock or tsc page) to our guests we need to > know TSC frequency and when it changes. > > Hyper-V Top Level Functional Specification (as of v5.0b) wrongly specifies > EAX:BIT(12) of CPUID:0x40000009 as the feature identification bit. The > right one to check is EAX:BIT(13) of CPUID:0x40000003. I was assured that > the fix in on the way. > > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
