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

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Message-ID: <20180228164822.GA12531@rkaganb.sw.ru>
Date:   Wed, 28 Feb 2018 19:48:22 +0300
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.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>,
        Andrey Smetanin <asmetanin@...tuozzo.com>,
        "Denis V . Lunev" <den@...nvz.org>
Subject: Re: [PATCH 1/3] x86/kvm/hyper-v: add reenlightenment MSRs support

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?

> 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>