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

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Date:   Wed, 28 Feb 2018 18:43:00 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Roman Kagan <rkagan@...tuozzo.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        "Denis V . Lunev" <den@...nvz.org>
Subject: Re: [PATCH 1/3] x86/kvm/hyper-v: add reenlightenment MSRs support

Roman Kagan <rkagan@...tuozzo.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@...hat.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@...tuozzo.com>

Thanks!

-- 
  Vitaly