Re: [PATCH 3/3] i386: Make sure kvm_arch_set_tsc_khz() succeeds on migration when 'hv-reenlightenment' was exposed

[Paolo Bonzini <pbonzini@redhat.com>]

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 <vkuznets@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).

Paolo