linux-kernel - Re: [PATCH v2 1/2] KVM: nVMX: don't use vcpu->arch.efer when checking host state on nested state load

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Message-ID: <e48b533f-8930-ab48-cbc3-660e2827b031@redhat.com>
Date:   Tue, 16 Nov 2021 11:03:30 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jim Mattson <jmattson@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2 1/2] KVM: nVMX: don't use vcpu->arch.efer when checking
 host state on nested state load

On 11/15/21 16:50, Sean Christopherson wrote:
>    When loading nested state, don't use check vcpu->arch.efer to get the
>    L1 host's 64-bit vs. 32-bit state and don't check it for consistency
>    with respect to VM_EXIT_HOST_ADDR_SPACE_SIZE, as register state in vCPU
>    may be stale when KVM_SET_NESTED_STATE is called and conceptually does
>    not exist.  When the CPU is in non-root mode, i.e. when restoring L2
>    state in KVM, there is no snapshot of L1 host state, it is (conditionally)
>    loaded on VM-Exit.  E.g. EFER is either preserved on exit, loaded from the
>    VMCS (vmcs12 in this case), or loaded from the MSR load list.
> 
>    Use vmcs12.VM_EXIT_HOST_ADDR_SPACE_SIZE to determine the target mode of
>    the L1 host, as it is the source of truth in this case.  Perform the EFER
>    vs. vmcs12.VM_EXIT_HOST_ADDR_SPACE_SIZE consistency check only on VM-Enter,
>    as conceptually there's no "current" L1 EFER to check.
> 
>    Note, KVM still checks vmcs12.HOST_EFER for consistency if
>    if vmcs12.VM_EXIT_LOAD_IA32_EFER is set, i.e. this skips only the check
>    against current vCPU state, which does not exist, when loading nested state.

Queued with some further edits and nested_vmx_check_address_state_size 
renamed to nested_vmx_check_address_*space*_size.

I think the "!!" are best left in place though, because "!!(a & b)" is 
idiomatic. Comparing "!(a & b)" would leave the reader wondering about 
the inversion, and "(bool)(a & b)" is just too ugly and magic.  The 
compiler anyway converts the "!!" to "!= 0" very early on, and never 
performs back-to-back logical NOTs.

Paolo