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

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Date:   Mon, 13 Jun 2022 16:57:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Anirudh Rayabharam <anrayabh@...ux.microsoft.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 Mon, Jun 13, 2022, Paolo Bonzini wrote:
> On 6/13/22 18:16, Anirudh Rayabharam wrote:
> > +	if (!kvm_has_tsc_control)
> > +		msrs->secondary_ctls_high &= ~SECONDARY_EXEC_TSC_SCALING;
> > +
> >   	msrs->secondary_ctls_low = 0;
> >   	msrs->secondary_ctls_high &=
> >   		SECONDARY_EXEC_DESC |
> > @@ -6667,8 +6670,7 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
> >   		SECONDARY_EXEC_RDRAND_EXITING |
> >   		SECONDARY_EXEC_ENABLE_INVPCID |
> >   		SECONDARY_EXEC_RDSEED_EXITING |
> > -		SECONDARY_EXEC_XSAVES |
> > -		SECONDARY_EXEC_TSC_SCALING;
> > +		SECONDARY_EXEC_XSAVES;
> >   	/*
> 
> This is wrong because it _always_ disables SECONDARY_EXEC_TSC_SCALING,
> even if kvm_has_tsc_control == true.
> 
> That said, I think a better implementation of this patch is to just add
> a version of evmcs_sanitize_exec_ctrls that takes a struct
> nested_vmx_msrs *, and call it at the end of nested_vmx_setup_ctl_msrs like
> 
> 	evmcs_sanitize_nested_vmx_vsrs(msrs);

Any reason not to use the already sanitized vmcs_config?  I can't think of any
reason why the nested path should blindly use the raw MSR values from hardware.