On Thu, Mar 18, 2021 at 05:38:00PM +0100, Vitaly Kuznetsov wrote: > Paolo Bonzini <pbonz...@redhat.com> writes: > > > On 18/03/21 17:02, Vitaly Kuznetsov wrote: > >> KVM doesn't fully support Hyper-V reenlightenment notifications on > >> migration. In particular, it doesn't support emulating TSC frequency > >> of the source host by trapping all TSC accesses so unless TSC scaling > >> is supported on the destination host and KVM_SET_TSC_KHZ succeeds, it > >> is unsafe to proceed with migration. > >> > >> Normally, we only require KVM_SET_TSC_KHZ to succeed when 'user_tsc_khz' > >> was set and just 'try' KVM_SET_TSC_KHZ without otherwise. > >> > >> Introduce a new vmstate section (which is added when the guest has > >> reenlightenment feature enabled) and add env.tsc_khz to it. We already > >> have env.tsc_khz packed in 'cpu/tsc_khz' but we don't want to be dependent > >> on the section order. > >> > >> Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com> > > > > Could we instead fail to load the reenlightenment section if > > user_tsc_khz was not set? This seems to be user (well, management) > > error really, since reenlightenment has to be enabled manually (or with > > hv-passthrough which blocks migration too).
Seems to match the strategy of the patchset... > Yes, we certainly could do that but what's the added value of > user_tsc_khz which upper layer will have to set explicitly (probably to > the tsc frequency of the source host anyway)? Yes. I think what happened was "evolution": 1) Added support to set tsc frequency (with hardware multiplier) in KVM, so add -tsc-khz VAL (kHz) option to KVM. 2) Scaling is enabled only if -tsc-khz VAL is supplied. 3) libvirt switches to using -tsc-khz HVAL, where HVAL it retrieves from KVM_GET_TSC_KHZ of newly created KVM_CREATE_VM instance. It could have been done inside qemu instead. > In case we just want to avoid calling KVM_SET_TSC_KHZ twice, we can probably > achieve that by > adding a CPU flag or something. Avoid calling KVM_SET_TSC_KHZ twice ? Don't see why you would avoid that.
