linux-kernel - Re: [PATCH v2] kvm: nVMX: Introduce KVM_CAP

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Date:   Mon, 9 Apr 2018 12:24:34 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     KarimAllah Ahmed <karahmed@...zon.de>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v2] kvm: nVMX: Introduce KVM_CAP_STATE

On Mon, Apr 9, 2018 at 1:37 AM, KarimAllah Ahmed <karahmed@...zon.de> wrote:

> +       /*
> +        * Force a nested exit that guarantees that any state capture
> +        * afterwards by any IOCTLs (MSRs, etc) will not capture a mix of L1
> +        * and L2 state.
> +        *
> +        * One example where that would lead to an issue is the TSC DEADLINE
> +        * MSR vs the guest TSC. If the L2 guest is running, the guest TSC will
> +        * be the L2 TSC while the TSC deadline MSR will contain the L1 TSC
> +        * deadline MSR. That would lead to a very large (and wrong) "expire"
> +        * diff when LAPIC is initialized during instance restore (i.e. the
> +        * instance will appear to have hanged!).
> +        */

This sounds like a bug in the virtualization of IA32_TSC_DEADLINE.
Without involving save/restore, what happens if L2 sets
IA32_TSC_DEADLINE (and L1 permits it via the MSR permission bitmap)?
The IA32_TSC_DEADLINE MSR is always specified with respect to L1's
time domain.