Re: [PATCH 1/3] x86/kvm/hyper-v: add reenlightenment MSRs support

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Roman Kagan <rkagan@virtuozzo.com> writes:

> On Wed, Feb 28, 2018 at 02:43:59PM +0100, Vitaly Kuznetsov wrote:
>> Nested Hyper-V/Windows guest running on top of KVM will use TSC page
>> clocksource in two cases:
>> - L0 exposes invariant TSC (CPUID.80000007H:EDX[8]).
>> - L0 provides Hyper-V Reenlightenment support (CPUID.40000003H:EAX[13]).
>> 
>> Exposing invariant TSC effectively blocks migration to hosts with different
>> TSC frequencies,
>
> I wonder if TSC scaling on the destination host doesn't allow to relax
> this requirement?

I don't see why it wouldn't, Skylake+ should be fine (but this, of
course, will limit possible destination hosts to those supporting the
feature -- even if no other CPU features are required).

>> providing reenlightenment support will be needed when we
>> start migrating nested workloads.
>> 
>> Implement rudimentary support for reenlightenment MSRs. For now, these are
>> just read/write MSRs with no effect.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  arch/x86/include/asm/kvm_host.h |  4 ++++
>>  arch/x86/kvm/hyperv.c           | 21 +++++++++++++++++++++
>>  arch/x86/kvm/x86.c              | 12 +++++++++++-
>>  3 files changed, 36 insertions(+), 1 deletion(-)
>
> Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>

Thanks!

-- 
  Vitaly