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

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Message-ID: <YrMenI1mTbqA9MaR@anrayabh-desk>
Date:   Wed, 22 Jun 2022 19:22:28 +0530
From:   Anirudh Rayabharam <anrayabh@...ux.microsoft.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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 Wed, Jun 22, 2022 at 10:00:29AM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> 
> > On Tue, Jun 14, 2022, Anirudh Rayabharam wrote:
> >> On Mon, Jun 13, 2022 at 04:57:49PM +0000, Sean Christopherson wrote:
> 
> ...
> 
> >> > 
> >> > 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.
> >> 
> >> vmcs_config has the sanitized exec controls. But how do we construct MSR
> >> values using them?
> >
> > I was thinking we could use the sanitized controls for the allowed-1 bits, and then
> > take the required-1 bits from the CPU.  And then if we wanted to avoid the redundant
> > RDMSRs in a follow-up patch we could add required-1 fields to vmcs_config.
> >
> > Hastily constructed and compile-tested only, proceed with caution :-)
> 
> Independently from "[PATCH 00/11] KVM: VMX: Support TscScaling and
> EnclsExitingBitmap whith eVMCS" which is supposed to fix the particular
> TSC scaling issue, I like the idea to make nested_vmx_setup_ctls_msrs()
> use both allowed-1 and required-1 bits from vmcs_config. I'll pick up
> the suggested patch and try to construct something for required-1 bits.

I tried this patch today but it causes some regression which causes
/dev/kvm to be unavailable in L1. I didn't get a chance to look into it
closely but I am guessing it has something to do with the fact that
vmcs_config reflects the config that L0 chose to use rather than what is
available to use. So constructing allowed-1 MSR bits based on what bits
are set in exec controls maybe isn't correct.


Thanks!

	- Anirudh.