linux-kernel - Re: [PATCH] Revert "KVM: x86: WARN and reject loading KVM if NX is supported but not enabled"

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Date:   Wed, 21 Jul 2021 21:28:22 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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] Revert "KVM: x86: WARN and reject loading KVM if NX is
 supported but not enabled"

On Tue, Jul 13, 2021, Yu Zhang wrote:
> On Mon, Jul 12, 2021 at 02:36:53PM +0000, Sean Christopherson wrote:
> > On Mon, Jul 12, 2021, Yu Zhang wrote:
> > > Why do we need EFER in that case? Thanks! :)
> > 
> > Because as you rightly remembered above, KVM always uses PAE paging for the guest,
> > even when the host is !PAE.  And KVM also requires EFER.NX=1 for the guest when
> > using shadow paging to handle a potential SMEP and !WP case.  
> 
> Just saw this in update_transition_efer(), which now enables efer.nx in shadow
> unconditionally. But I guess the host kernel still needs to set efer.nx for
> !PAE(e.g. in head_32.S),

Yep, and that's what I messed up.

> because the guest may not touch efer at all. Is this correct?

KVM doesn't require EFER.NX "because the guest may not touch efer at all", it
requires EFER.NX to handle scenarios where KVM needs to make a guest page
!EXECUTABLE even if EFER is not exposed to the guest (thanks to SMEP && !WP).