linux-kernel - Re: [PATCH] KVM: nVMX: Don't expose TSC scaling to L1 when on Hyper-V

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Message-ID: <ec63cf3c-d312-01b7-671b-02729bddfa32@redhat.com>
Date:   Tue, 14 Jun 2022 19:28:10 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Anirudh Rayabharam <anrayabh@...ux.microsoft.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Ilias Stamatis <ilstam@...zon.com>,
        Maxim Levitsky <mlevitsk@...hat.com>, mail@...rudhrb.com,
        kumarpraveen@...ux.microsoft.com, wei.liu@...nel.org,
        robert.bradford@...el.com, liuwe@...rosoft.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: Don't expose TSC scaling to L1 when on Hyper-V

On 6/14/22 17:13, Anirudh Rayabharam wrote:
>>> Sanitize at the end might not work because I see some cases in
>>> nested_vmx_setup_ctls_msrs() where we want to expose some things to L1
>>> even though the hardware doesn't support it.
>>
>> Yes, but these will never include eVMCS-unsupported features.
>
> How are you so sure?
> 
> For example, SECONDARY_EXEC_SHADOW_VMCS is unsupported in eVMCS but in
> nested_vmx_setup_ctls_msrs() we do:
> 
> 6675         /*
> 6676          * We can emulate "VMCS shadowing," even if the hardware
> 6677          * doesn't support it.
> 6678          */
> 6679         msrs->secondary_ctls_high |=
> 6680                 SECONDARY_EXEC_SHADOW_VMCS;
> 
> If we sanitize this out it might cause some regression right?

Yes, you're right, shadow VMCS is special: it is not supported by 
enlightened VMCS, but it is emulated rather than virtualized. 
Therefore, if L1 does not use the enlightened VMCS, it can indeed use 
shadow VMCS.

Paolo