linux-kernel - Re: [PATCH 0/2] KVM: nVMX: vmcs.SYSENTER optimization and "fix"

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Date:   Tue, 28 Apr 2020 16:45:25 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] KVM: nVMX: vmcs.SYSENTER optimization and "fix"

On Tue, Apr 28, 2020 at 4:10 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Patch 1 is a "fix" for handling SYSENTER_EIP/ESP in L2 on a 32-bit vCPU.
> The primary motivation is to provide consistent behavior after patch 2.
>
> Patch 2 is essentially a re-submission of a nested VMX optimization to
> avoid redundant VMREADs to the SYSENTER fields in the nested VM-Exit path.
>
> After patch 2 and without patch 1, KVM would end up with weird behavior
> where L1 and L2 would only see 32-bit values for their own SYSENTER_E*P
> MSRs, but L1 could see a 64-bit value for L2's MSRs.
>
> Sean Christopherson (2):
>   KVM: nVMX: Truncate writes to vmcs.SYSENTER_EIP/ESP for 32-bit vCPU
>   KVM: nVMX: Drop superfluous VMREAD of vmcs02.GUEST_SYSENTER_*
>
>  arch/x86/kvm/vmx/nested.c |  4 ----
>  arch/x86/kvm/vmx/vmx.c    | 18 ++++++++++++++++--
>  2 files changed, 16 insertions(+), 6 deletions(-)

It seems like this could be fixed more generally by truncating
natural-width fields on 32-bit vCPUs in handle_vmwrite(). However,
that also would imply that we can't shadow any natural-width fields on
a 32-bit vCPU.