linux-kernel - Re: [PATCH] KVM: nVMX: Always enable TSC scaling for L2 when it was enabled for L1

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Date:   Wed, 13 Jul 2022 09:44:59 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH] KVM: nVMX: Always enable TSC scaling for L2 when it was
 enabled for L1

Dongli Zhang <dongli.zhang@...cle.com> writes:

> Hi Vitaly,
>
> On 7/12/22 6:50 AM, Vitaly Kuznetsov wrote:
>> Windows 10/11 guests with Hyper-V role (WSL2) enabled are observed to
>> hang upon boot or shortly after when a non-default TSC frequency was
>> set for L1. The issue is observed on a host where TSC scaling is
>
> Would you mind helping clarify if it is L1 or L2 that hangs?
>
> The commit message "Windows 10/11 guests with Hyper-V role (WSL2)" confuses me
> if it is L1 or L2 (perhaps due to my lack of knowledge on hyper-v) that hangs.
>

I think it's L2 but I'm not sure: there's no easy way to interract with
L1 (Hyper-V) directly, all the interfaces (UI, network,..) are handled
by L2 (Windows). Prior to the observed 'hang' Hyper-V (L1) programms
synthetic timer in KVM too far in the future but my guess is that it's
doing that on Windows' (L2) behalf, basically just relaying the
request. The issue only shows up with 'hv-reenlightenment' +
'hv-frequencies' (in QEMU's terms) features as in this case both Hyper-V
(L1) and Windows (L2) trust the information about TSC frequency from KVM
but the information turns out to be incorrect for Windows (L2).

-- 
Vitaly