linux-kernel - Re: [PATCH] KVM: x86: Allow guest to set EFER.NX=1 on non-PAE 32-bit kernels

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Date:   Fri, 6 Aug 2021 13:58:00 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Allow guest to set EFER.NX=1 on non-PAE 32-bit
 kernels

On 05/08/21 20:38, Sean Christopherson wrote:
> Fast forward to today, and KVM has long since stopped running the guest
> with the host's EFER.NX.  Not only does KVM context switch EFER if
> host.EFER.NX=1 && guest.EFER.NX=0, KVM also forces host.EFER.NX=0 &&
> guest.EFER.NX=1 when using shadow paging (to emulate SMEP).  Furthermore,
> the entire motivation for the restriction was made obsolete over a decade
> ago when Intel added dedicated host and guest EFER fields in the VMCS
> (Nehalem timeframe), which reduced the overhead of context switching EFER
> from 400+ cycles (2 * WRMSR + 1 * RDMSR) to a mere ~2 cycles.

Both pretty good points. :)  Queued, thanks.

Paolo