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

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Date:   Tue, 12 Jul 2022 17:27:53 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: Always enable TSC scaling for L2 when it was
 enabled for L1

On Tue, 2022-07-12 at 15:50 +0200, 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
> supported. The problem appears to be that Windows doesn't use TSC
> frequency for its guests even when the feature is advertised and KVM
> filters SECONDARY_EXEC_TSC_SCALING out when creating L2 controls from
> L1's. This leads to L2 running with the default frequency (matching
> host's) while L1 is running with an altered one.

Ouch.

I guess that needs a Fixes tag?

Fixes: d041b5ea93352b ("KVM: nVMX: Enable nested TSC scaling")

Also this is thankfully Intel specific, because in AMD you can't enable
TSC scaling - there is just an MSR with default value of 1.0,
which one can change if TSC scaling is supported in CPUID.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
Best regards,
	Maxim Levitsky


> 
> Keep SECONDARY_EXEC_TSC_SCALING in secondary exec controls for L2 when
> it was set for L1. TSC_MULTIPLIER is already correctly computed and
> written by prepare_vmcs02().
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 778f82015f03..bfa366938c49 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2284,7 +2284,6 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
>                                   SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY |
>                                   SECONDARY_EXEC_APIC_REGISTER_VIRT |
>                                   SECONDARY_EXEC_ENABLE_VMFUNC |
> -                                 SECONDARY_EXEC_TSC_SCALING |
>                                   SECONDARY_EXEC_DESC);
>  
>                 if (nested_cpu_has(vmcs12,