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 17:33:39 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...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

Maxim Levitsky <mlevitsk@...hat.com> writes:

> 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

^^^ scaling ^^^

>> 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")
>

I dismissed that because prior to d041b5ea93352b SECONDARY_EXEC_TSC_SCALING
was filtered out in nested_vmx_setup_ctls_msrs() but now I think I was
wrong, SECONDARY_EXEC_TSC_SCALING was likely kept in VMCS02 regardless
of that. Will add in v2.

> 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>

Thanks!

> 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,
>
>

-- 
Vitaly